Add "mock" compilation feature to dart2js and use it to improve test coverage.

dart/tools/testing/dart/test_suite.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.
 
 /**
  * Classes and methods for enumerating and preparing tests.
  *
  * This library includes:
  *
  * - Creating tests by listing all the Dart files in certain directories,
@@ skipping 1155 matching lines @@
       args.add('--enable_asserts');
       args.add("--enable_type_checks");
     }
     if (configuration["component"] == "dart2js"
         || configuration["component"] == "legium") {
       args.add("--verbose");
       args.add("--leg");
       if (configuration["host_checked"]) {
         args.add("--vm_flags=--enable_asserts --enable_type_checks");
       }
+      if (configuration["component"] == "dart2js") {
+        args.add("--allow-mock-compilation");
+      }
     }
     return args;
   }
 
   static bool isBrowserComponent(String component) =>
       const <String>['dartium',
                      'frogium',
                      'legium',
                      'webdriver'].some((x) => x == component);
 }
@@ skipping 42 matching lines @@
  * $noCrash tests are expected to be flaky but not crash
  * $pass tests are expected to pass
  * $failOk tests are expected to fail that we won't fix
  * $fail tests are expected to fail that we should fix
  * $crash tests are expected to crash that we should fix
  * $timeout tests are allowed to timeout
 """;
     print(report);
    }
 }