Improve performance of dart2js StringBuffer.

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

Index: dart/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart b/dart/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
index 18c8a1eb828390d94ac177545da8086fec929eda..4297edba4303838d3ab52ff5f133c2a827428861 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
@@ -11,7 +11,8 @@ import 'dart:_js_helper' show checkNull,
                               JSSyntaxRegExp,
                               Primitives,
                               TypeImpl,
-                              stringJoinUnchecked;
+                              stringJoinUnchecked,
+                              JsStringBuffer;
 
 // Patch for 'print' function.
 patch void print(var object) {
@@ -265,3 +266,9 @@ patch class RegExp {
 patch bool identical(Object a, Object b) {
   return Primitives.identicalImplementation(a, b);
 }
+
+patch class StringBuffer {
+  patch factory StringBuffer([Object content = ""]) {
+    return new JsStringBuffer(content);
+  }
+}