Remove JavaScriptBackend from ClosedWorldBase

pkg/compiler/lib/src/universe/resolution_world_builder.dart

 // Copyright (c) 2017, 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.
 
 part of world_builder;
 
 abstract class ResolutionWorldBuilder implements WorldBuilder, OpenWorld {
   /// Set of all local functions in the program. Used by the mirror tracking
   /// system to find all live closure instances.
   Iterable<Local> get localFunctions;
@@ skipping 778 matching lines @@
 class KernelResolutionWorldBuilder extends ResolutionWorldBuilderBase {
   KernelResolutionWorldBuilder(
       ElementEnvironment elementEnvironment,
       CommonElements commonElements,
       NativeBasicData nativeBasicData,
       SelectorConstraintsStrategy selectorConstraintsStrategy)
       : super(elementEnvironment, commonElements, nativeBasicData,
             selectorConstraintsStrategy);
 
   @override
-  ClosedWorld closeWorld(DiagnosticReporter reporter) {
+  ClosedWorld closeWorld() {
     // TODO(johnniwinther): Implement this.
-    return null;
+    return _closedWorldCache = new KernelClosedWorld(
+        commonElements: _commonElements,
+        // TODO(johnniwinther): Compute these.
+        constantSystem: null,
+        nativeData: null,
+        interceptorData: null,
+        backendUsage: null,
+        resolutionWorldBuilder: this,
+        functionSetBuilder: _allFunctions,
+        allTypedefs: _allTypedefs,
+        mixinUses: _mixinUses,
+        // TODO(johnniwinther): Compute this.
+        typesImplementedBySubclasses: null,
+        classHierarchyNodes: _classHierarchyNodes,
+        classSets: _classSets);
   }
 
   @override
   void registerClass(ClassEntity cls) {
     throw new UnimplementedError('KernelResolutionWorldBuilder.registerClass');
   }
 }