Remove warnings from type mirrors and add argument checks.

tests/compiler/dart2js/exit_code_test.dart

 // Copyright (c) 2013, 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 the exit code of dart2js in case of exceptions, fatal errors, errors,
 // warnings, etc.
 
 
 import 'dart:async';
 import 'dart:io' show Platform;
@@ skipping 200 matching lines @@
   });
 }
 
 Future testExitCodes(String marker, Map<String,int> expectedExitCodes) {
   return Future.forEach(expectedExitCodes.keys, (String type) {
     return testExitCode(marker, type, expectedExitCodes[type]);
   });
 }
 
 void main() {
+  // TODO(johnniwinther): implement this test for unchecked mode.
   bool isCheckedMode = false;
   assert(isCheckedMode = true);
   Expect.isTrue(isCheckedMode, 'This test must be run in checked mode.');
 
   const beforeRun = const {
     '': 0,
     'NoSuchMethodError': 253,
     'assert': 253,
     'invariant': 253
   };
@@ skipping 27 matching lines @@
     int countResults(Map runType) {
       return runType.length *
              tests.values.where((r) => r == runType).length;
     }
 
     Expect.equals(countResults(beforeRun) + countResults(duringRun),
                   checkedResults);
     asyncEnd();
   });
 }