Simplify dart2js StringBuffer constructor to be initializer-only.

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

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library _js_helper;
 
 import 'shared/embedded_names.dart' show
     ALL_CLASSES,
     GET_ISOLATE_TAG,
     INTERCEPTED_NAMES,
@@ skipping 808 matching lines @@
       }
     }
     throw new RangeError.range(charCode, 0, 0x10ffff);
   }
 
   static String stringConcatUnchecked(String string1, String string2) {
     return JS_STRING_CONCAT(string1, string2);
   }
 
   static String flattenString(String str) {
-    return JS('', "#.charCodeAt(0) == 0 ? # : #", str, str, str);
+    return JS('String', "#.charCodeAt(0) == 0 ? # : #", str, str, str);
   }
 
   static String getTimeZoneName(receiver) {
     // Firefox and Chrome emit the timezone in parenthesis.
     // Example: "Wed May 16 2012 21:13:00 GMT+0200 (CEST)".
     // We extract this name using a regexp.
     var d = lazyAsJsDate(receiver);
     List match = JS('JSArray|Null', r'/\((.*)\)/.exec(#.toString())', d);
     if (match != null) return match[1];
 
@@ skipping 2568 matching lines @@
   throw new MainError("No top-level function named 'main'.");
 }
 
 void badMain() {
   throw new MainError("'main' is not a function.");
 }
 
 void mainHasTooManyParameters() {
   throw new MainError("'main' expects too many parameters.");
 }