Refactor BackendHelpers to be reusageable with KernelWorldBuilder

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 199 matching lines @@
     }
     return false;
   }
 
   /// Called to register that [element] is statically known to be used. Any
   /// backend specific [WorldImpact] of this is returned.
   WorldImpact registerUsedElement(MemberElement element,
           {bool forResolution}) =>
       const WorldImpact();
 
-  /// This method is called immediately after the [LibraryElement] [library] has
-  /// been created.
-  void onLibraryCreated(LibraryElement library) {}
-
   /// This method is called immediately after the [library] and its parts have
   /// been scanned.
   Future onLibraryScanned(LibraryElement library, LibraryLoader loader) {
     // TODO(johnniwinther): Move this to [JavaScriptBackend].
     if (!compiler.serialization.isDeserialized(library)) {
       if (canLibraryUseNative(library)) {
         library.forEachLocalMember((Element element) {
           if (element.isClass) {
             checkNativeAnnotation(compiler, element);
           }
@@ skipping 78 matching lines @@
   /// Called when the compiler starts running the codegen enqueuer. The
   /// [WorldImpact] of enabled backend features is returned.
   WorldImpact onCodegenStart(ClosedWorld closedWorld) => const WorldImpact();
 
   /// Called when code generation has been completed.
   void onCodegenEnd() {}
 
   // Does this element belong in the output
   bool shouldOutput(Element element) => true;
 
-  FunctionElement helperForBadMain() => null;
+  MethodElement helperForBadMain() => null;
 
-  FunctionElement helperForMissingMain() => null;
+  MethodElement helperForMissingMain() => null;
 
-  FunctionElement helperForMainArity() => null;
+  MethodElement helperForMainArity() => null;
 
   /// Computes the [WorldImpact] of calling [mainMethod] as the entry point.
   WorldImpact computeMainImpact(MethodElement mainMethod,
           {bool forResolution}) =>
       const WorldImpact();
 
   /// Returns the location of the patch-file associated with [libraryName]
   /// resolved from [plaformConfigUri].
   ///
   /// Returns null if there is none.
@@ skipping 171 matching lines @@
   /// Returns `true` if [cls] is an intercepted class.
   // TODO(johnniwinther): Rename this to `isInterceptedClass`.
   bool isInterceptorClass(ClassEntity cls);
 
   /// Returns `true` if [cls] is a native class.
   bool isNativeClass(ClassEntity element);
 
   /// Returns `true` if [element] is a native member of a native class.
   bool isNativeMember(MemberEntity element);
 }