Fixes for sunflower

test/generated_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 'dart:_js_embedded_names' show
     ALL_CLASSES,
     GET_ISOLATE_TAG,
     INTERCEPTED_NAMES,
@@ skipping 564 matching lines @@
 
   static int objectHashCode(object) {
     int hash = JS('int|Null', r'#.$identityHash', object);
     if (hash == null) {
       hash = JS('int', '(Math.random() * 0x3fffffff) | 0');
       JS('void', r'#.$identityHash = #', object, hash);
     }
     return JS('int', '#', hash);
   }
 
-  static _throwFormatException(String string) {
+  static Null _throwFormatException(String string) {
     throw new FormatException(string);
   }
 
   static int parseInt(String source,
                       int radix,
                       int handleError(String source)) {
     if (handleError == null) handleError = _throwFormatException;
 
     checkString(source);
     var match = JS('JSExtendableArray|Null',
@@ skipping 2859 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.");
 }