Improve performance of dart2js StringBuffer.

dart/sdk/lib/_internal/compiler/implementation/lib/string_helper.dart

Index: dart/sdk/lib/_internal/compiler/implementation/lib/string_helper.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/lib/string_helper.dart b/dart/sdk/lib/_internal/compiler/implementation/lib/string_helper.dart
index 6e4dccfb2b057aceb67c705d27ef2c06a2dfab45..8ac1fa4b3697dc67316b50acfae212a23fc56f77 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/lib/string_helper.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/lib/string_helper.dart
@@ -202,3 +202,33 @@ stringReplaceFirstUnchecked(receiver, from, to) {
 stringJoinUnchecked(array, separator) {
   return JS('String', r'#.join(#)', array, separator);
 }
+
+class JsStringBuffer implements StringBuffer {
+  String _contents;
+
+  JsStringBuffer(content)
+      : _contents = (content is String) ? content : '$content';
+
+  int get length => _contents.length;
+
+  bool get isEmpty => length == 0;
+
+  void add(Object obj) {
+    _contents = JS('String', '# + #', _contents,
+                   (obj is String) ? obj : '$obj');
+  }
+
+  void addAll(Iterable objects) {
+    for (Object obj in objects) add(obj);
+  }
+
+  void addCharCode(int charCode) {
+    add(new String.fromCharCodes([charCode]));
+  }
+
+  void clear() {
+    _contents = "";
+  }
+
+  String toString() => _contents;
+}