Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/simple_inferrer_postfix_prefix_test.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.
 
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 
 import 'compiler_helper.dart';
 import 'type_mask_test_helper.dart';
 
@@ skipping 53 matching lines @@
 
 void main() {
   Uri uri = new Uri(scheme: 'source');
   var compiler = compilerFor(TEST, uri);
   asyncTest(() => compiler.run(uri).then((_) {
         var typesInferrer = compiler.globalInference.typesInferrerInternal;
         var closedWorld = typesInferrer.closedWorld;
         var commonMasks = closedWorld.commonMasks;
 
         checkReturnInClass(String className, String methodName, type) {
-          var cls = findElement(compiler, className);
+          dynamic cls = findElement(compiler, className);
           var element = cls.lookupLocalMember(methodName);
           Expect.equals(
               type,
               simplify(
                   typesInferrer.getReturnTypeOfElement(element), closedWorld),
               methodName);
         }
 
         var subclassOfInterceptor = commonMasks.interceptorType;
 
@@ skipping 12 matching lines @@
         checkReturnInClass(
             'B', 'returnDynamic1', const TypeMask.nonNullEmpty());
         checkReturnInClass(
             'B', 'returnDynamic2', const TypeMask.nonNullEmpty());
         checkReturnInClass(
             'B', 'returnDynamic3', const TypeMask.nonNullEmpty());
         checkReturnInClass(
             'B', 'returnDynamic4', const TypeMask.nonNullEmpty());
       }));
 }