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

pkg/compiler/lib/src/js_backend/backend.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 js_backend.backend;
 
 import 'dart:async' show Future;
 
 import 'package:js_runtime/shared/embedded_names.dart' as embeddedNames;
 
@@ skipping 579 matching lines @@
   final NativeData nativeData = new NativeData();
 
   final BackendHelpers helpers;
   final BackendImpacts impacts;
 
   final JSFrontendAccess frontend;
 
   JavaScriptBackend(Compiler compiler,
       {bool generateSourceMap: true,
       bool useStartupEmitter: false,
-      bool useNewSourceInfo: false})
+      bool useNewSourceInfo: false,
+      bool useKernel: false})
       : namer = determineNamer(compiler),
         oneShotInterceptors = new Map<jsAst.Name, Selector>(),
         interceptedElements = new Map<String, Set<Element>>(),
         rti = new _RuntimeTypes(compiler),
         rtiEncoder = new _RuntimeTypesEncoder(compiler),
         specializedGetInterceptors = new Map<jsAst.Name, Set<ClassElement>>(),
         annotations = new Annotations(compiler),
         this.sourceInformationStrategy = generateSourceMap
             ? (useNewSourceInfo
                 ? new PositionSourceInformationStrategy()
                 : const StartEndSourceInformationStrategy())
             : const JavaScriptSourceInformationStrategy(),
         helpers = new BackendHelpers(compiler),
         impacts = new BackendImpacts(compiler),
         frontend = new JSFrontendAccess(compiler),
         super(compiler) {
     emitter = new CodeEmitterTask(
         compiler, namer, generateSourceMap, useStartupEmitter);
     typeVariableHandler = new TypeVariableHandler(compiler);
     customElementsAnalysis = new CustomElementsAnalysis(this);
     lookupMapAnalysis = new LookupMapAnalysis(this, reporter);
     jsInteropAnalysis = new JsInteropAnalysis(this);
 
     noSuchMethodRegistry = new NoSuchMethodRegistry(this);
     constantCompilerTask = new JavaScriptConstantTask(compiler);
     impactTransformer = new JavaScriptImpactTransformer(this);
     patchResolverTask = new PatchResolverTask(compiler);
+    // TODO(sigmund): pass along the kernel flag.
     functionCompiler = new SsaFunctionCompiler(this, sourceInformationStrategy);
     serialization = new JavaScriptBackendSerialization(this);
   }
 
   ConstantSystem get constantSystem => constants.constantSystem;
 
   DiagnosticReporter get reporter => compiler.reporter;
 
   CoreClasses get coreClasses => compiler.coreClasses;
 
@@ skipping 2558 matching lines @@
 
   @override
   void onImpactUsed(ImpactUseCase impactUse) {
     if (impactUse == DeferredLoadTask.IMPACT_USE && !supportSerialization) {
       // TODO(johnniwinther): Allow emptying when serialization has been
       // performed.
       resolution.emptyCache();
     }
   }
 }