Add flag to dart2js to configure whether to use the kernel lowering.

pkg/compiler/lib/src/dart2js.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.
 
 library dart2js.cmdline;
 
 import 'dart:async' show EventSink, Future;
 import 'dart:convert' show UTF8, LineSplitter;
 import 'dart:io' show exit, File, FileMode, Platform, stdin, stderr;
 
@@ skipping 306 matching lines @@
       passThrough(Flags.suppressWarnings);
     }),
     new OptionHandler(Flags.fatalWarnings, passThrough),
     new OptionHandler(Flags.suppressHints, (_) {
       showHints = false;
     }),
     // TODO(sigmund): remove entirely after Dart 1.20
     new OptionHandler(
         '--output-type=dart|--output-type=dart-multi|--output-type=js',
         setOutputType),
+    new OptionHandler(Flags.useKernel, passThrough),
     new OptionHandler(Flags.noFrequencyBasedMinification, passThrough),
     new OptionHandler(Flags.verbose, setVerbose),
     new OptionHandler(Flags.version, (_) => wantVersion = true),
     new OptionHandler('--library-root=.+', setLibraryRoot),
     new OptionHandler('--out=.+|-o.*', setOutput, multipleArguments: true),
     new OptionHandler(Flags.allowMockCompilation, passThrough),
     new OptionHandler(Flags.fastStartup, passThrough),
     new OptionHandler(Flags.genericMethodSyntax, passThrough),
     new OptionHandler(Flags.initializingFormalAccess, passThrough),
     new OptionHandler('${Flags.minify}|-m', implyCompilation),
@@ skipping 691 matching lines @@
   @override
   void close() {
     // Do nothing.
   }
 
   @override
   void addError(errorEvent, [StackTrace stackTrace]) {
     // Ignore
   }
 }