Move hasRuntimeTypeSupport, hasIsolateSupport, and hasFunctionApplySupport to JavaScriptBackend.

pkg/compiler/lib/src/common/backend_api.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.backend_api;
 
 import 'dart:async' show Future;
 
 import '../common.dart';
 import '../common/codegen.dart' show CodegenImpact;
@@ skipping 296 matching lines @@
   /// [recentClasses], but every class seen by the [enqueuer] will be present in
   /// [recentClasses] at least once.
   bool onQueueEmpty(Enqueuer enqueuer, Iterable<ClassEntity> recentClasses) {
     return true;
   }
 
   /// Called after the queue is closed. [onQueueEmpty] may be called multiple
   /// times, but [onQueueClosed] is only called once.
   void onQueueClosed() {}
 
-  /// Called when the compiler starts running the codegen enqueuer.
-  void onCodegenStart() {}
+  /// Called when the compiler starts running the codegen enqueuer. The
+  /// [WorldImpact] of enabled backend features is returned.
+  WorldImpact onCodegenStart() => const WorldImpact();
 
   // Does this element belong in the output
   bool shouldOutput(Element element) => true;
 
   FunctionElement helperForBadMain() => null;
 
   FunctionElement helperForMissingMain() => null;
 
   FunctionElement helperForMainArity() => null;
 
@@ skipping 83 matching lines @@
   ClassElement get typeImplementation;
   ClassElement get boolImplementation;
   ClassElement get nullImplementation;
   ClassElement get uint32Implementation;
   ClassElement get uint31Implementation;
   ClassElement get positiveIntImplementation;
   ClassElement get syncStarIterableImplementation;
   ClassElement get asyncFutureImplementation;
   ClassElement get asyncStarStreamImplementation;
 }