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 5ccfcfc55381a7d59057b21e1f110a357169be06..e7005aa30d5db50d09553a726e7037076b9c821a 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
@@ -244,9 +244,28 @@ patch bool identical(Object a, Object b) {
 }
 
 patch class StringBuffer {
-  patch factory StringBuffer([Object content = ""]) {
-    return new JsStringBuffer(content);
+  String _contents = "";
+
+  patch StringBuffer([Object content = ""]) {
+    if (content is String) {
+      _contents = content;
+    } else {
+      write(content);
+    }
   }
+
+  patch int get length => _contents.length;
+
+  patch void write(Object obj) {
+    String str = obj is String ? obj : "$obj";
+    _contents = Primitives.stringConcatUnchecked(_contents, str);
+  }
+
+  patch void clear() {
+    _contents = "";
+  }
+
+  patch String toString() => _contents;
 }
 
 patch class NoSuchMethodError {