Revert "Remove WorldImpact from caches when no longer needed."

pkg/compiler/lib/src/common/backend_api.dart

Index: pkg/compiler/lib/src/common/backend_api.dart
diff --git a/pkg/compiler/lib/src/common/backend_api.dart b/pkg/compiler/lib/src/common/backend_api.dart
index d2fa5ec96cc6139b48bad64caf247adb783a6ad7..ff49bcc14e03f2940696f79dbbb5feaf189ea33a 100644
--- a/pkg/compiler/lib/src/common/backend_api.dart
+++ b/pkg/compiler/lib/src/common/backend_api.dart
@@ -60,7 +60,6 @@ import '../tree/tree.dart' show
 import '../universe/call_structure.dart' show
     CallStructure;
 import '../universe/world_impact.dart' show
-    ImpactStrategy,
     WorldImpact;
 
 import 'codegen.dart' show
@@ -425,13 +424,6 @@ abstract class Backend {
   ///
   /// 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.