Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/const_exp_test.dart

 // Copyright (c) 2014, 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 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 import 'type_test_helper.dart';
 
 test(String constantInitializer, [String expectedOutput]) {
@@ skipping 14 matching lines @@
       static staticFunction() {}
     }
     const t = true;
     const f = false;
     const toplevelConstant = 0;
     toplevelFunction() {}
     const constant = $constantInitializer;
 """,
               expectNoWarningsOrErrors: true)
           .then((env) {
-        var element = env.getElement('constant');
+        dynamic element = env.getElement('constant');
         Expect.isNotNull(element, "Element 'constant' not found.");
         var constant = element.constant;
         var value = env.compiler.constants.getConstantValue(constant);
         Expect.isNotNull(constant, "No constant computed for '$element'.");
         Expect.equals(
             expectedOutput,
             constant.toDartText(),
             "Unexpected to string '${constant.toDartText()}' for constant "
             "'$constantInitializer' of value "
             "${value.toStructuredText()}");
@@ skipping 56 matching lines @@
         test('t ? t : f ? f ? 0 : 1 : 2'),
         test('t ? t ? t : t : t ? t : t'),
         test('t ? (t ? t : t) : (t ? t : t)', 't ? t ? t : t : t ? t : t'),
         test(
             'const [const <dynamic, dynamic>{0: true, "1": "c" "d"}, '
             'const Class(const Class<dynamic, dynamic>(toplevelConstant))]',
             'const [const {0: true, "1": "cd"}, '
             'const Class(const Class(toplevelConstant))]'),
       ], (f) => f()));
 }