Remove WorldImpact from caches when no longer needed.

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
@@ skipping 42 matching lines @@
 import '../patch_parser.dart' show
     checkNativeAnnotation, checkJsInteropAnnotation;
 import '../resolution/tree_elements.dart' show
     TreeElements;
 import '../tree/tree.dart' show
     Node,
     Send;
 import '../universe/call_structure.dart' show
     CallStructure;
 import '../universe/world_impact.dart' show
+    ImpactStrategy,
     WorldImpact;
 
 import 'codegen.dart' show
     CodegenWorkItem;
 import 'registry.dart' show
     Registry;
 import 'tasks.dart' show
     CompilerTask;
 import 'work.dart' show
     ItemCompilationContext;
@@ skipping 344 matching lines @@
   void registerForeignCall(Send node,
                            Element element,
                            CallStructure callStructure,
                            ForeignResolver resolver) {}
 
   /// Returns the location of the patch-file associated with [libraryName]
   /// resolved from [plaformConfigUri].
   ///
   /// Returns null if there is none.
   Uri resolvePatchUri(String libraryName, Uri plaformConfigUri);
+
+  /// Creates an impact strategy to use for compilation.
+  ImpactStrategy createImpactStrategy(
+      {bool supportDeferredLoad: true,
+       bool supportDumpInfo: true}) {
+    return const ImpactStrategy();
+  }
 }
 
 /// Interface for resolving calls to foreign functions.
 abstract class ForeignResolver {
   /// Returns the constant expression of [node], or `null` if [node] is not
   /// a constant expression.
   ConstantExpression getConstant(Node node);
 
   /// Registers [type] as instantiated.
   void registerInstantiatedType(InterfaceType type);
 
   /// Resolves [typeName] to a type in the context of [node].
   DartType resolveTypeFromString(Node node, String typeName);
 }
 
 /// Backend transformation methods for the world impacts.
 class ImpactTransformer {
   /// Transform the [ResolutionImpact] into a [WorldImpact] adding the
   /// backend dependencies for features used in [worldImpact].
   WorldImpact transformResolutionImpact(ResolutionImpact worldImpact) {
     return worldImpact;
   }
 
   /// Transform the [CodegenImpact] into a [WorldImpact] adding the
   /// backend dependencies for features used in [worldImpact].
   WorldImpact transformCodegenImpact(CodegenImpact worldImpact) {
     return worldImpact;
   }
 }