Revert "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 7d8473f8e2cd2cc0e7df31947bddf4be43cfaeda..09e31da11d0734d44ea381e10de9c76a926089b2 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
@@ -244,25 +244,7 @@ patch bool identical(Object a, Object b) {
 }
 
 patch class StringBuffer {
-  String _contents = "";
-
-  patch StringBuffer([Object content = ""]) {
-    if (content is String) {
-      _contents = content;
-    } else {
-      write(content);
-    }
-  }
-
-  patch int get length => _contents.length;
-
-  patch void _write(String str) {
-    _contents = Primitives.stringConcat(_contents, str);
-  }
-
-  patch void clear() {
-    _contents = "";
+  patch factory StringBuffer([Object content = ""]) {
+    return new JsStringBuffer(content);
   }
-
-  patch String toString() => _contents;
 }