Small optimization in String.subString

Change-Id: Ib72d4903c47dc928f817595270cbfa476c098f5f
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 d0e6bdb..7db2125 100644
--- a/user/super/com/google/gwt/emul/java/lang/String.java
+++ b/user/super/com/google/gwt/emul/java/lang/String.java
@@ -683,7 +683,7 @@
   }
 
   public String substring(int beginIndex) {
-    return asNativeString().substr(beginIndex, length() - beginIndex);
+    return asNativeString().substr(beginIndex);
   }
 
   public String substring(int beginIndex, int endIndex) {
@@ -763,6 +763,7 @@
     public native int lastIndexOf(String str);
     public native int lastIndexOf(String str, int start);
     public native String replace(NativeRegExp regex, String replace);
+    public native String substr(int beginIndex);
     public native String substr(int beginIndex, int len);
     public native String toLocaleLowerCase();
     public native String toLocaleUpperCase();