Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/js_parser_statements_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:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
 import 'mock_compiler.dart';
 import 'package:compiler/src/js/js.dart' as jsAst;
 import 'package:compiler/src/js/js.dart' show js;
@@ skipping 10 matching lines @@
 
 Future testError(String statement, arguments, [String expect = ""]) {
   return new Future.sync(() {
     bool doCheck(exception) {
       String message = '$exception';
       Expect.isTrue(message.contains(expect), '"$message" contains "$expect"');
       return true;
     }
 
     void action() {
-      jsAst.Node node = js.statement(statement, arguments);
+      js.statement(statement, arguments);
     }
 
     Expect.throws(action, doCheck);
   });
 }
 
 // Function declaration and named function.
 const NAMED_FUNCTION_1 = r'''
 function foo/*function declaration*/() {
   return function harry/*named function*/() { return #; }
@@ skipping 472 matching lines @@
             [],
             """
 if (a) {
   while (true)
     if (b)
       foo1();
 } else
   foo2();"""),
       ]));
 }