Add StringSink and update StringBuffer.

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

Index: sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart b/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
index 09e31da11d0734d44ea381e10de9c76a926089b2..ae47f86c89edab0c0285adeadf6152f6e79e540b 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
@@ -244,7 +244,17 @@ patch bool identical(Object a, Object b) {
 }
 
 patch class StringBuffer {
-  patch factory StringBuffer([Object content = ""]) {
-    return new JsStringBuffer(content);
+  String _contents = "";
+
+  patch int get length => _contents.length;
+
+  patch void _write(String str) {
+    _contents = JS('String', '# + #', _contents, str);
   }
+
+  patch void clear() {
+    _contents = "";
+  }
+
+  patch String toString() => _contents;
 }