fix language_tests.js to use strict mode

test/codegen/language/asyncstar_throw_in_catch_test.dart

 // Copyright (c) 2015, 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";
 
 class Tracer {
   final String expected;
   final String name;
   String _trace = "";
   int counter = 0;
 
   Tracer(this.expected, [this.name]);
 
   void trace(msg) {
     if (name != null) {
-      print("Tracing $name: $msg");
+      // Commented out, see https://github.com/dart-lang/dev_compiler/issues/278
+      //print("Tracing $name: $msg");
     }
     _trace += msg;
     counter++;
   }
 
   void done() {
     Expect.equals(expected, _trace);
   }
 }
 
@@ skipping 92 matching lines @@
   await runTest(foo1, "abcdYefC", null, true);
   await runTest(foo2, "abcX", "Error", false);
   await runTest(foo3, "abcYX", "Error", false);
   await runTest(foo4, "abcdYeYfX", "Error2", false);
 }
 
 
 void main() {
   asyncTest(test);
 }