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

pkg/compiler/lib/src/compiler.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.
 
 library dart2js.compiler_base;
 
 import 'dart:async' show EventSink, Future;
 
 import '../compiler_new.dart' as api;
 import 'cache_strategy.dart' show CacheStrategy;
@@ skipping 238 matching lines @@
     // for global dependencies.
     globalDependencies = new GlobalDependencyRegistry(this);
 
     if (makeBackend != null) {
       backend = makeBackend(this);
     } else {
       js_backend.JavaScriptBackend jsBackend = new js_backend.JavaScriptBackend(
           this,
           generateSourceMap: options.generateSourceMap,
           useStartupEmitter: options.useStartupEmitter,
-          useNewSourceInfo: options.useNewSourceInfo);
+          useNewSourceInfo: options.useNewSourceInfo,
+          useKernel: options.useKernel);
       backend = jsBackend;
     }
 
     if (options.dumpInfo && options.useStartupEmitter) {
       throw new ArgumentError(
           '--dump-info is not supported with the fast startup emitter');
     }
 
     tasks = [
       dietParser =
@@ skipping 1963 matching lines @@
   _ElementScanner(this.scanner);
   void scanLibrary(LibraryElement library) => scanner.scanLibrary(library);
   void scanUnit(CompilationUnitElement unit) => scanner.scan(unit);
 }
 
 class _EmptyEnvironment implements Environment {
   const _EmptyEnvironment();
 
   String valueOf(String key) => null;
 }