Improve performance of dart2js StringBuffer.

dart/sdk/lib/core/string_buffer.dart

 // Copyright (c) 2011, 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);
+  external factory StringBuffer([Object 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();
 }
-
-class _StringBufferImpl implements StringBuffer {
-
-  List<String> _buffer;
-  int _length;
-
-  /// Creates the string buffer with an initial content.
-  _StringBufferImpl(Object content) {
-    clear();
-    add(content);
-  }
-
-  /// Returns the length of the buffer.
-  int get length => _length;
-
-  bool get isEmpty => _length == 0;
-
-  /// Adds [obj] to the buffer.
-  void add(Object obj) {
-    // TODO(srdjan): The following four lines could be replaced by
-    // '$obj', but apparently this is too slow on the Dart VM.
-    String str = obj.toString();
-    if (str is !String) {
-      throw new ArgumentError('toString() did not return a string');
-    }
-    if (str.isEmpty) return;
-    _buffer.add(str);
-    _length += str.length;
-  }
-
-  /// Adds all items in [objects] to the buffer.
-  void addAll(Iterable objects) {
-    for (Object obj in objects) add(obj);
-  }
-
-  /// Adds the string representation of [charCode] to the buffer.
-  void addCharCode(int charCode) {
-    add(new String.fromCharCodes([charCode]));
-  }
-
-  /// Clears the string buffer.
-  void clear() {
-    _buffer = new List<String>();
-    _length = 0;
-  }
-
-  /// Returns the contents of buffer as a concatenated string.
-  String toString() {
-    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;
-  }
-}