diff --git a/user/super/com/google/gwt/emul/java/lang/Character.java b/user/super/com/google/gwt/emul/java/lang/Character.java
index ca0c93d..61db5e1 100644
--- a/user/super/com/google/gwt/emul/java/lang/Character.java
+++ b/user/super/com/google/gwt/emul/java/lang/Character.java
@@ -230,7 +230,7 @@
    * TODO: correct Unicode handling.
    */
   public static boolean isDigit(char c) {
-    return String.valueOf(c).nativeMatches(digitRegex());
+    return digitRegex().test(String.valueOf(c));
   }
 
   private static native NativeRegExp digitRegex() /*-{
@@ -245,7 +245,7 @@
    * TODO: correct Unicode handling.
    */
   public static boolean isLetter(char c) {
-    return String.valueOf(c).nativeMatches(leterRegex());
+    return leterRegex().test(String.valueOf(c));
   }
 
   private static native NativeRegExp leterRegex() /*-{
@@ -256,7 +256,7 @@
    * TODO: correct Unicode handling.
    */
   public static boolean isLetterOrDigit(char c) {
-    return String.valueOf(c).nativeMatches(leterOrDigitRegex());
+    return leterOrDigitRegex().test(String.valueOf(c));
   }
 
   private static native NativeRegExp leterOrDigitRegex() /*-{
@@ -296,11 +296,11 @@
   }
 
   public static boolean isWhitespace(char ch) {
-    return String.valueOf(ch).nativeMatches(whitespaceRegex());
+    return whitespaceRegex().test(String.valueOf(ch));
   }
 
   public static boolean isWhitespace(int codePoint) {
-    return String.fromCodePoint(codePoint).nativeMatches(whitespaceRegex());
+    return whitespaceRegex().test(String.fromCodePoint(codePoint));
   }
 
   // The regex would just be /\s/, but browsers handle non-breaking spaces inconsistently. Also,
diff --git a/user/super/com/google/gwt/emul/java/lang/String.java b/user/super/com/google/gwt/emul/java/lang/String.java
index 1e355e3..18bd4c0 100644
--- a/user/super/com/google/gwt/emul/java/lang/String.java
+++ b/user/super/com/google/gwt/emul/java/lang/String.java
@@ -210,13 +210,6 @@
     return replaceStr;
   }
 
-  private static native int compareTo(String thisStr, String otherStr) /*-{
-    if (thisStr == otherStr) {
-      return 0;
-    }
-    return thisStr < otherStr ? -1 : 1;
-  }-*/;
-
   private static Charset getCharset(String charsetName) throws UnsupportedEncodingException {
     try {
       return Charset.forName(charsetName);
@@ -382,15 +375,15 @@
 
   @Override
   public int compareTo(String other) {
-    return compareTo(this, other);
+    return JsUtils.compare(checkNotNull(this), checkNotNull(other));
   }
 
   public int compareToIgnoreCase(String other) {
-    return compareTo(toLowerCase(), other.toLowerCase());
+    return toLowerCase().compareTo(other.toLowerCase());
   }
 
   public String concat(String str) {
-    return this + str;
+    return checkNotNull(this) + checkNotNull(str);
   }
 
   public boolean contains(CharSequence s) {
@@ -421,18 +414,15 @@
   }
 
   public boolean equalsIgnoreCase(String other) {
-    return equalsIgnoreCase(this, other);
-  }
-
-  private static native boolean equalsIgnoreCase(String s, String other) /*-{
+    checkNotNull(this);
     if (other == null) {
       return false;
     }
-    if (s == other) {
+    if (equals(other)) {
       return true;
     }
-    return (s.length == other.length) && (s.toLowerCase() == other.toLowerCase());
-  }-*/;
+    return length() == other.length() && toLowerCase().equals(other.toLowerCase());
+  }
 
   public byte[] getBytes() {
     // default character set for GWT is UTF-8
@@ -450,7 +440,10 @@
   public void getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin) {
     checkCriticalStringBounds(srcBegin, srcEnd, length());
     checkCriticalStringBounds(dstBegin, dstBegin + (srcEnd - srcBegin), dst.length);
+    getChars0(srcBegin, srcEnd, dst, dstBegin);
+  }
 
+  private void getChars0(int srcBegin, int srcEnd, char[] dst, int dstBegin) {
     while (srcBegin < srcEnd) {
       dst[dstBegin++] = charAt(srcBegin++);
     }
@@ -476,9 +469,9 @@
   public int indexOf(String str, int startIndex) {
     return asNativeString().indexOf(str, startIndex);
   }
-  
+
   public String intern() {
-    return this;
+    return checkNotNull(this);
   }
 
   public boolean isEmpty() {
@@ -516,11 +509,7 @@
    */
   public boolean matches(String regex) {
     // We surround the regex with '^' and '$' because it must match the entire string.
-    return nativeMatches(new NativeRegExp("^(" + regex + ")$"));
-  }
-
-  boolean nativeMatches(NativeRegExp regex) {
-    return regex.test(this);
+    return new NativeRegExp("^(" + regex + ")$").test(this);
   }
 
   public int offsetByCodePoints(int index, int codePointOffset) {
@@ -529,13 +518,11 @@
 
   public boolean regionMatches(boolean ignoreCase, int toffset, String other,
       int ooffset, int len) {
-    if (other == null) {
-      throw new NullPointerException();
-    }
+    checkNotNull(other);
     if (toffset < 0 || ooffset < 0 || len <= 0) {
       return false;
     }
-    if (toffset + len > this.length() || ooffset + len > other.length()) {
+    if (toffset + len > length() || ooffset + len > other.length()) {
       return false;
     }
 
@@ -692,11 +679,11 @@
 
   @Override
   public CharSequence subSequence(int beginIndex, int endIndex) {
-    return this.substring(beginIndex, endIndex);
+    return substring(beginIndex, endIndex);
   }
 
   public String substring(int beginIndex) {
-    return asNativeString().substr(beginIndex, this.length() - beginIndex);
+    return asNativeString().substr(beginIndex, length() - beginIndex);
   }
 
   public String substring(int beginIndex, int endIndex) {
@@ -704,9 +691,9 @@
   }
 
   public char[] toCharArray() {
-    int n = this.length();
+    int n = length();
     char[] charArr = new char[n];
-    getChars(0, n, charArr, 0);
+    getChars0(0, n, charArr, 0);
     return charArr;
   }
 
@@ -833,15 +820,15 @@
   }
 
   protected static String $create(String other) {
-    return other;
+    return checkNotNull(other);
   }
 
   protected static String $create(StringBuffer sb) {
-    return String.valueOf(sb);
+    return sb.toString();
   }
 
   protected static String $create(StringBuilder sb) {
-    return String.valueOf(sb);
+    return sb.toString();
   }
 
   @JsMethod
diff --git a/user/super/com/google/gwt/emul/java/lang/StringBuffer.java b/user/super/com/google/gwt/emul/java/lang/StringBuffer.java
index 39be45d..91f154a 100644
--- a/user/super/com/google/gwt/emul/java/lang/StringBuffer.java
+++ b/user/super/com/google/gwt/emul/java/lang/StringBuffer.java
@@ -1,12 +1,12 @@
 /*
  * Copyright 2008 Google Inc.
- * 
+ *
  * Licensed under the Apache License, Version 2.0 (the "License"); you may not
  * use this file except in compliance with the License. You may obtain a copy of
  * the License at
- * 
+ *
  * http://www.apache.org/licenses/LICENSE-2.0
- * 
+ *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
  * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
@@ -15,9 +15,11 @@
  */
 package java.lang;
 
+import static javaemul.internal.InternalPreconditions.checkNotNull;
+
 /**
  * A fast way to create strings using multiple appends.
- * 
+ *
  * This class is an exact clone of {@link StringBuilder} except for the name.
  * Any change made to one should be mirrored in the other.
  */
@@ -28,7 +30,7 @@
   }
 
   public StringBuffer(CharSequence s) {
-    super(String.valueOf(s));
+    super(s.toString());
   }
 
   /**
@@ -41,7 +43,7 @@
   }
 
   public StringBuffer(String s) {
-    super(s);
+    super(checkNotNull(s));
   }
 
   public StringBuffer append(boolean x) {
diff --git a/user/super/com/google/gwt/emul/java/lang/StringBuilder.java b/user/super/com/google/gwt/emul/java/lang/StringBuilder.java
index 78bcd03..da705ef 100644
--- a/user/super/com/google/gwt/emul/java/lang/StringBuilder.java
+++ b/user/super/com/google/gwt/emul/java/lang/StringBuilder.java
@@ -1,12 +1,12 @@
 /*
  * Copyright 2008 Google Inc.
- * 
+ *
  * Licensed under the Apache License, Version 2.0 (the "License"); you may not
  * use this file except in compliance with the License. You may obtain a copy of
  * the License at
- * 
+ *
  * http://www.apache.org/licenses/LICENSE-2.0
- * 
+ *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
  * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
@@ -15,9 +15,11 @@
  */
 package java.lang;
 
+import static javaemul.internal.InternalPreconditions.checkNotNull;
+
 /**
  * A fast way to create strings using multiple appends.
- * 
+ *
  * This class is an exact clone of {@link StringBuffer} except for the name.
  * Any change made to one should be mirrored in the other.
  */
@@ -28,7 +30,7 @@
   }
 
   public StringBuilder(CharSequence s) {
-    super(String.valueOf(s));
+    super(s.toString());
   }
 
   /**
@@ -41,7 +43,7 @@
   }
 
   public StringBuilder(String s) {
-    super(s);
+    super(checkNotNull(s));
   }
 
   public StringBuilder append(boolean x) {
diff --git a/user/super/com/google/gwt/emul/javaemul/internal/JsUtils.java b/user/super/com/google/gwt/emul/javaemul/internal/JsUtils.java
index 981f224..8796b61 100644
--- a/user/super/com/google/gwt/emul/javaemul/internal/JsUtils.java
+++ b/user/super/com/google/gwt/emul/javaemul/internal/JsUtils.java
@@ -20,6 +20,10 @@
  */
 public class JsUtils {
 
+  public static native int compare(String a, String b) /*-{
+    return a == b ? 0 : (a < b ? -1 : 1);
+  }-*/;
+
   public static native boolean isFinite(double d) /*-{
     return isFinite(d);
   }-*/;
diff --git a/user/test/com/google/gwt/emultest/java/lang/StringTest.java b/user/test/com/google/gwt/emultest/java/lang/StringTest.java
index b9f06aa..3b556ce 100644
--- a/user/test/com/google/gwt/emultest/java/lang/StringTest.java
+++ b/user/test/com/google/gwt/emultest/java/lang/StringTest.java
@@ -15,6 +15,7 @@
  */
 package com.google.gwt.emultest.java.lang;
 
+import com.google.gwt.core.client.JavaScriptException;
 import com.google.gwt.junit.client.GWTTestCase;
 import com.google.gwt.testing.TestUtils;
 
@@ -31,22 +32,13 @@
  */
 public class StringTest extends GWTTestCase {
 
-  static volatile boolean TRUE = true;
-
-  private static <T> T hideFromCompiler(T value) {
-    return TRUE ? value : null;
-  }
-
   @Override
   public String getModuleName() {
     return "com.google.gwt.emultest.EmulSuite";
   }
 
-  /*
-   * TODO: needs rewriting to avoid compiler optimizations.
-   */
   public void testCharAt() {
-    assertEquals("abc".charAt(1), 'b');
+    assertEquals(hideFromCompiler("abc").charAt(1), 'b');
   }
 
   public void testCodePoint() {
@@ -103,6 +95,30 @@
     assertEquals(1, nonBmpChar.codePointCount(0, 2));
   }
 
+  public void testCompareTo() {
+    assertEquals(0, hideFromCompiler("a").compareTo("a"));
+    assertEquals(-1, hideFromCompiler("a").compareTo("b"));
+    assertEquals(1, hideFromCompiler("b").compareTo("a"));
+
+    try {
+      returnNull().compareTo("");
+      fail();
+    } catch (NullPointerException e) {
+      // expected
+    } catch (JavaScriptException e) {
+      // expected
+    }
+
+    try {
+      returnNull().compareTo(returnNull());
+      fail();
+    } catch (NullPointerException e) {
+      // expected
+    } catch (JavaScriptException e) {
+      // expected
+    }
+  }
+
   public void testConcat() {
     String abc = String.valueOf(new char[] {'a', 'b', 'c'});
     String def = String.valueOf(new char[] {'d', 'e', 'f'});
@@ -118,6 +134,24 @@
     assertEquals("abcd", s + c);
     s += c;
     assertEquals("abcd", s);
+
+    try {
+      returnNull().concat("");
+      fail();
+    } catch (NullPointerException e) {
+      // expected
+    } catch (JavaScriptException e) {
+      // expected
+    }
+
+    try {
+      hideFromCompiler("").concat(returnNull());
+      fail();
+    } catch (NullPointerException e) {
+      // expected
+    } catch (JavaScriptException e) {
+      // expected
+    }
   }
 
   public void testConstructor() {
@@ -147,6 +181,22 @@
     sb = new StringBuilder();
     sb.appendCodePoint(0x10400);
     assertEquals("\uD801\uDC00", new String(sb));
+
+    try {
+      new String(returnNull());
+    } catch (NullPointerException e) {
+      // expected
+    } catch (JavaScriptException e) {
+      // expected
+    }
+
+    try {
+      new String(hideFromCompiler((StringBuilder) null));
+    } catch (NullPointerException e) {
+      // expected
+    } catch (JavaScriptException e) {
+      // expected
+    }
   }
 
   public void testConstructorBytes() {
@@ -265,26 +315,22 @@
     }
   }
 
-  /*
-   * TODO: needs rewriting to avoid compiler optimizations. (StringBuffer tests
-   * are ok)
-   */
   public void testContains() {
     // at the beginning
-    assertTrue("abcdef".contains("ab"));
-    assertTrue("abcdef".contains(new StringBuffer("ab")));
+    assertTrue(hideFromCompiler("abcdef").contains("ab"));
+    assertTrue(hideFromCompiler("abcdef").contains(new StringBuffer("ab")));
     // at the end
-    assertTrue("abcdef".contains("ef"));
-    assertTrue("abcdef".contains(new StringBuffer("ef")));
+    assertTrue(hideFromCompiler("abcdef").contains("ef"));
+    assertTrue(hideFromCompiler("abcdef").contains(new StringBuffer("ef")));
     // in the middle
-    assertTrue("abcdef".contains("cd"));
-    assertTrue("abcdef".contains(new StringBuffer("cd")));
+    assertTrue(hideFromCompiler("abcdef").contains("cd"));
+    assertTrue(hideFromCompiler("abcdef").contains(new StringBuffer("cd")));
     // the same
-    assertTrue("abcdef".contains("abcdef"));
-    assertTrue("abcdef".contains(new StringBuffer("abcdef")));
+    assertTrue(hideFromCompiler("abcdef").contains("abcdef"));
+    assertTrue(hideFromCompiler("abcdef").contains(new StringBuffer("abcdef")));
     // not present
-    assertFalse("abcdef".contains("z"));
-    assertFalse("abcdef".contains(new StringBuffer("z")));
+    assertFalse(hideFromCompiler("abcdef").contains("z"));
+    assertFalse(hideFromCompiler("abcdef").contains(new StringBuffer("z")));
   }
 
   public void testEndsWith() {
@@ -295,38 +341,64 @@
   }
 
   public void testEquals() {
-    assertFalse("ABC".equals("abc"));
-    assertFalse("abc".equals("ABC"));
-    assertTrue("abc".equals("abc"));
-    assertTrue("ABC".equals("ABC"));
-    assertFalse("AbC".equals("aBC"));
-    assertFalse("AbC".equals("aBC"));
-    assertTrue("".equals(""));
-    assertFalse("".equals(null));
+    assertFalse(hideFromCompiler("ABC").equals("abc"));
+    assertFalse(hideFromCompiler("abc").equals("ABC"));
+    assertTrue(hideFromCompiler("abc").equals("abc"));
+    assertTrue(hideFromCompiler("ABC").equals("ABC"));
+    assertFalse(hideFromCompiler("AbC").equals("aBC"));
+    assertFalse(hideFromCompiler("AbC").equals("aBC"));
+    assertTrue(hideFromCompiler("").equals(""));
+    assertFalse(hideFromCompiler("").equals(null));
 
-    // Randomize the string to avoid static eval.
-    double r = Math.random();
-    assertTrue((r + "").equals(r + ""));
-    assertFalse((r + "ABC").equals(r + "abc"));
-    assertFalse((r + "abc").equals(r + "ABC"));
-    assertTrue((r + "abc").equals(r + "abc"));
-    assertTrue((r + "ABC").equals(r + "ABC"));
-    assertFalse((r + "AbC").equals(r + "aBC"));
-    assertFalse((r + "AbC").equals(r + "aBC"));
+    // TODO: String.equals does not have NPE check
+/*
+    try {
+      returnNull().equals("other");
+      fail();
+    } catch (NullPointerException e) {
+      // expected
+    } catch (JavaScriptException e) {
+      // expected
+    }
+
+    try {
+      returnNull().equals(returnNull());
+      fail();
+    } catch (NullPointerException e) {
+      // expected
+    } catch (JavaScriptException e) {
+      // expected
+    }
+*/
   }
 
-  /*
-   * TODO: needs rewriting to avoid compiler optimizations.
-   */
   public void testEqualsIgnoreCase() {
-    assertTrue("ABC".equalsIgnoreCase("abc"));
-    assertTrue("abc".equalsIgnoreCase("ABC"));
-    assertTrue("abc".equalsIgnoreCase("abc"));
-    assertTrue("ABC".equalsIgnoreCase("ABC"));
-    assertTrue("AbC".equalsIgnoreCase("aBC"));
-    assertTrue("AbC".equalsIgnoreCase("aBC"));
-    assertTrue("".equalsIgnoreCase(""));
-    assertFalse("".equalsIgnoreCase(null));
+    assertTrue(hideFromCompiler("ABC").equalsIgnoreCase("abc"));
+    assertTrue(hideFromCompiler("abc").equalsIgnoreCase("ABC"));
+    assertTrue(hideFromCompiler("abc").equalsIgnoreCase("abc"));
+    assertTrue(hideFromCompiler("ABC").equalsIgnoreCase("ABC"));
+    assertTrue(hideFromCompiler("AbC").equalsIgnoreCase("aBC"));
+    assertTrue(hideFromCompiler("AbC").equalsIgnoreCase("aBC"));
+    assertTrue(hideFromCompiler("").equalsIgnoreCase(""));
+    assertFalse(hideFromCompiler("").equalsIgnoreCase(null));
+
+    try {
+      returnNull().equalsIgnoreCase("other");
+      fail();
+    } catch (NullPointerException e) {
+      // expected
+    } catch (JavaScriptException e) {
+      // expected
+    }
+
+    try {
+      returnNull().equalsIgnoreCase(returnNull());
+      fail();
+    } catch (NullPointerException e) {
+      // expected
+    } catch (JavaScriptException e) {
+      // expected
+    }
   }
 
   public void testGetBytesAscii() {
@@ -440,6 +512,15 @@
       // get hashes again to verify the values are constant for a given string
       assertEquals(expectedHash, testStrings[i].hashCode());
     }
+
+    try {
+      returnNull().hashCode();
+      fail();
+    } catch (NullPointerException e) {
+      // expected
+    } catch (JavaScriptException e) {
+      // expected
+    }
   }
 
   public void testIndexOf() {
@@ -451,6 +532,15 @@
     assertEquals(haystack.indexOf('a', 1), -1);
     assertEquals(haystack.indexOf("bc"), 1);
     assertEquals(haystack.indexOf(""), 0);
+
+    try {
+      returnNull().indexOf("");
+      fail();
+    } catch (NullPointerException e) {
+      // expected
+    } catch (JavaScriptException e) {
+      // expected
+    }
   }
 
   public void testIntern() {
@@ -458,6 +548,15 @@
     String s2 = String.valueOf(new char[] {'a', 'b', 'c', 'd', 'e', 'f'});
     assertTrue("strings not equal", s1.equals(s2));
     assertSame("interns are not the same reference", s1.intern(), s2.intern());
+
+    try {
+      returnNull().intern();
+      fail();
+    } catch (NullPointerException e) {
+      // expected
+    } catch (JavaScriptException e) {
+      // expected
+    }
   }
 
   public void testLastIndexOf() {
@@ -479,21 +578,18 @@
     assertEquals(4, cat.length());
   }
 
-  /*
-   * TODO: needs rewriting to avoid compiler optimizations.
-   */
   public void testLowerCase() {
-    assertEquals("abc", "AbC".toLowerCase());
-    assertEquals("abc", "abc".toLowerCase());
-    assertEquals("", "".toLowerCase());
+    assertEquals("abc", hideFromCompiler("AbC").toLowerCase());
+    assertEquals("abc", hideFromCompiler("abc").toLowerCase());
+    assertEquals("", hideFromCompiler("").toLowerCase());
 
-    assertEquals("abc", "AbC".toLowerCase(Locale.US));
-    assertEquals("abc", "abc".toLowerCase(Locale.US));
-    assertEquals("", "".toLowerCase(Locale.US));
+    assertEquals("abc", hideFromCompiler("AbC").toLowerCase(Locale.US));
+    assertEquals("abc", hideFromCompiler("abc").toLowerCase(Locale.US));
+    assertEquals("", hideFromCompiler("").toLowerCase(Locale.US));
 
-    assertEquals("abc", "AbC".toLowerCase(Locale.getDefault()));
-    assertEquals("abc", "abc".toLowerCase(Locale.getDefault()));
-    assertEquals("", "".toLowerCase(Locale.getDefault()));
+    assertEquals("abc", hideFromCompiler("AbC").toLowerCase(Locale.getDefault()));
+    assertEquals("abc", hideFromCompiler("abc").toLowerCase(Locale.getDefault()));
+    assertEquals("", hideFromCompiler("").toLowerCase(Locale.getDefault()));
   }
 
   public void testMatch() {
@@ -687,14 +783,10 @@
     assertFalse(haystack.startsWith(haystack + "j"));
   }
 
-  /*
-   * TODO: needs rewriting to avoid compiler optimizations.
-   */
   public void testSubstring() {
-    String haystack = "abcdefghi";
-    assertEquals("cd", haystack.substring(2, 4));
-    assertEquals("bc", "abcdef".substring(1, 3));
-    assertEquals("bcdef", "abcdef".substring(1));
+    assertEquals("cd", hideFromCompiler("abcdefghi").substring(2, 4));
+    assertEquals("bc", hideFromCompiler("abcdef").substring(1, 3));
+    assertEquals("bcdef", hideFromCompiler("abcdef").substring(1));
   }
 
   public void testToCharArray() {
@@ -712,7 +804,7 @@
     assertEquals("concat with str.toString()", "0abcd", "0" + str.toString() + "d");
 
     // issue 4301
-    Object s = TRUE ? "abc" : createJavaScriptObject();
+    Object s = hideFromCompiler("abc");
     assertSame("s same as s.toString()", s, s.toString());
   }
 
@@ -751,21 +843,18 @@
     trimRightAssertEquals("\u2029abc\u00a0","\u2029abc\u00a0");
   }
 
-  /*
-   * TODO: needs rewriting to avoid compiler optimizations.
-   */
   public void testUpperCase() {
-    assertEquals("ABC", "AbC".toUpperCase());
-    assertEquals("ABC", "abc".toUpperCase());
-    assertEquals("", "".toUpperCase());
+    assertEquals("ABC", hideFromCompiler("AbC").toUpperCase());
+    assertEquals("ABC", hideFromCompiler("abc").toUpperCase());
+    assertEquals("", hideFromCompiler("").toUpperCase());
 
-    assertEquals("ABC", "AbC".toUpperCase(Locale.US));
-    assertEquals("ABC", "abc".toUpperCase(Locale.US));
-    assertEquals("", "".toUpperCase(Locale.US));
+    assertEquals("ABC", hideFromCompiler("AbC").toUpperCase(Locale.US));
+    assertEquals("ABC", hideFromCompiler("abc").toUpperCase(Locale.US));
+    assertEquals("", hideFromCompiler("").toUpperCase(Locale.US));
 
-    assertEquals("ABC", "AbC".toUpperCase(Locale.getDefault()));
-    assertEquals("ABC", "abc".toUpperCase(Locale.getDefault()));
-    assertEquals("", "".toUpperCase(Locale.getDefault()));
+    assertEquals("ABC", hideFromCompiler("AbC").toUpperCase(Locale.getDefault()));
+    assertEquals("ABC", hideFromCompiler("abc").toUpperCase(Locale.getDefault()));
+    assertEquals("", hideFromCompiler("").toUpperCase(Locale.getDefault()));
   }
 
   /*
@@ -811,28 +900,20 @@
     }
   }
 
-  private String returnNull() {
+  private <T> T hideFromCompiler(T value) {
     if (Math.random() < -1) {
       // Can never happen, but fools the compiler enough not to optimize this call.
       fail();
-      return "";
     }
-    return null;
+    return value;
+  }
+
+  private String returnNull() {
+    return hideFromCompiler(null);
   }
 
   private String toS(char from) {
     return Character.toString(from);
   }
 
-  private static Object createJavaScriptObject() {
-    if (TestUtils.isJvm()) {
-      return new Object();
-    }
-    return createJavaScriptObject0();
-  }
-
-  private static native Object createJavaScriptObject0() /*-{
-    return {};
-  }-*/;
-
 }