Support checked mode for return type.

tests/language/return_in_loop_test.dart

 // Copyright (c) 2012, 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.
-// Test for a leg bug where the live environment was not computed
+// Test for a dart2js bug where the live environment was not computed
 // right.
 
 class A {
   foo() {
     var x = 0;
     while (true) {
       if (true) {
         return 42;
       } else {
       }
       x = bar();
     }
   }
   bar() => 1;
 }
 
 main() {
   Expect.equals(42, new A().foo());
 }