Strong mode cleaning of many dart2js tests.

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, errors, warnings, etc.
 
 import 'dart:async';
 import 'dart:io' show Platform;
 
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 
 import 'package:compiler/compiler_new.dart' as api;
-import 'package:compiler/src/common/backend_api.dart';
 import 'package:compiler/src/common/codegen.dart';
 import 'package:compiler/src/common/resolution.dart';
 import 'package:compiler/src/compile_time_constants.dart';
 import 'package:compiler/src/compiler.dart';
 import 'package:compiler/src/dart2js.dart' as entry;
 import 'package:compiler/src/diagnostics/diagnostic_listener.dart';
 import 'package:compiler/src/diagnostics/invariant.dart';
 import 'package:compiler/src/diagnostics/messages.dart';
 import 'package:compiler/src/diagnostics/spannable.dart';
 import 'package:compiler/src/apiimpl.dart' as apiimpl;
@@ skipping 283 matching lines @@
 
       expected =
           _expectedExitCode(beforeRun: tests[marker], fatalWarnings: true);
       totalExpectedErrors += expected.length;
       await testExitCodes(marker, expected, ['--fatal-warnings']);
     }
 
     Expect.equals(totalExpectedErrors, checkedResults);
   });
 }