Improve performance of dart2js StringBuffer.

dart/runtime/lib/string_buffer_patch.dart

-// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-part of dart.core;
-
-/**
- * The StringBuffer class is useful for concatenating strings
- * efficiently. Only on a call to [toString] are the strings
- * concatenated to a single String.
- */
-abstract class StringBuffer {
-
-  /// Creates the string buffer with an initial content.
-  factory StringBuffer([Object content = ""]) => new _StringBufferImpl(content);
-
-  /// Returns the length of the buffer.
-  int get length;
-
-  // Returns whether the buffer is empty.
-  bool get isEmpty;
-
-  /// Converts [obj] to a string and adds it to the buffer.
-  void add(Object obj);
-
-  /// Adds the string representation of [charCode] to the buffer.
-  void addCharCode(int charCode);
-
-  /// Adds all items in [objects] to the buffer.
-  void addAll(Iterable objects);
-
-  /// Clears the string buffer.
-  void clear();
-
-  /// Returns the contents of buffer as a concatenated string.
-  String toString();
+patch /* abstract */ class StringBuffer {
+  /* patch */ factory StringBuffer([Object content = ""])
+    => new _StringBufferImpl(content);
 }
 
 class _StringBufferImpl implements StringBuffer {
 
   List<String> _buffer;
   int _length;
 
   /// Creates the string buffer with an initial content.
   _StringBufferImpl(Object content) {
     clear();
@@ skipping 39 matching lines @@
     if (_buffer.length == 0) return "";
     if (_buffer.length == 1) return _buffer[0];
     String result = Strings.concatAll(_buffer);
     _buffer.clear();
     _buffer.add(result);
     // Since we track the length at each add operation, there is no
     // need to update it in this function.
     return result;
   }
 }