Overriding mark and reset methods in emulated StringReader.

Change-Id: If7b9a6dbd324d8571d1c103bc4a9f66d86e8e3d6

user/super/com/google/gwt/emul/java/io/StringReader.java

diff --git a/user/super/com/google/gwt/emul/java/io/StringReader.java b/user/super/com/google/gwt/emul/java/io/StringReader.java
index 08d3241..d0e8a0f 100644
--- a/user/super/com/google/gwt/emul/java/io/StringReader.java
+++ b/user/super/com/google/gwt/emul/java/io/StringReader.java
@@ -15,12 +15,15 @@
  */
 package java.io;
 
+import static javaemul.internal.InternalPreconditions.checkArgument;
+
 /**
  * Reads characters from a string.
  */
 public class StringReader extends Reader {
   private final String text;
   private int position;
+  private int mark;
 
   /**
    * Constructs a reader which will read from the given string.
@@ -45,4 +48,20 @@
     position += length;
     return length;
   }
+
+  @Override
+  public boolean markSupported() {
+    return true;
+  }
+
+  @Override
+  public void mark(int readAheadLimit) throws IOException {
+    checkArgument(readAheadLimit >= 0);
+    mark = position;
+  }
+
+  @Override
+  public void reset() throws IOException {
+    position = mark;
+  }
 }