Revert "Add StringSink and update StringBuffer."

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

Index: sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart b/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
index 627eb9559d557e0b4d41d26e492957c85a4d08d8..0fbff9c6ac7296a29e8f6a9642bb77b865f2baef 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
@@ -345,10 +345,6 @@ class Primitives {
     return _fromCharCodeApply(charCodes);
   }
 
-  static String stringConcat(String string1, String string2) {
-    return JS('String', r'# + #', string1, string2);
-  }
-
   static String getTimeZoneName(receiver) {
     // When calling toString on a Date it will emit the timezone in parenthesis.
     // Example: "Wed May 16 2012 21:13:00 GMT+0200 (CEST)".