Remove WorldImpact from caches when no longer needed.

pkg/compiler/lib/src/js_backend/backend.dart

Index: pkg/compiler/lib/src/js_backend/backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend.dart b/pkg/compiler/lib/src/js_backend/backend.dart
index 0fed323ab40d412424e6dbe745c8ab04901a84c5..6898d6ace17dfb5a108f84d1ce990b53e7e52536 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -1602,7 +1602,10 @@ class JavaScriptBackend extends Backend {
     }
 
     generatedCode[element] = functionCompiler.compile(work);
-    return impactTransformer.transformCodegenImpact(work.registry.worldImpact);
+    WorldImpact worldImpact =
+        impactTransformer.transformCodegenImpact(work.registry.worldImpact);
+    compiler.dumpInfoTask.registerImpact(element, worldImpact);
+    return worldImpact;
   }
 
   native.NativeEnqueuer nativeResolutionEnqueuer(Enqueuer world) {
@@ -2611,6 +2614,17 @@ class JavaScriptBackend extends Backend {
     if (patchLocation == null) return null;
     return platformConfigUri.resolve(patchLocation);
   }
+
+  @override
+  ImpactStrategy createImpactStrategy(
+      {bool supportDeferredLoad: true,
+       bool supportDumpInfo: true}) {
+    return new JavaScriptImpactStrategy(
+        resolution,
+        compiler.dumpInfoTask,
+        supportDeferredLoad: supportDeferredLoad,
+        supportDumpInfo: supportDumpInfo);
+  }
 }
 
 /// Handling of special annotations for tests.
@@ -3079,3 +3093,46 @@ class Dependency {
 
   const Dependency(this.constant, this.annotatedElement);
 }
+
+class JavaScriptImpactStrategy extends ImpactStrategy {
+  final Resolution resolution;
+  final DumpInfoTask dumpInfoTask;
+  final bool supportDeferredLoad;
+  final bool supportDumpInfo;
+
+  JavaScriptImpactStrategy(this.resolution,
+                           this.dumpInfoTask,
+                           {this.supportDeferredLoad,
+                            this.supportDumpInfo});
+
+  @override
+  void visitImpact(Element element,
+                   WorldImpact impact,
+                   WorldImpactVisitor visitor,
+                   ImpactUseCase impactUse) {
+    // TODO(johnniwinther): Compute the application strategy once for each use.
+    if (impactUse == ResolutionEnqueuer.IMPACT_USE) {
+      if (supportDeferredLoad) {
+        impact.apply(visitor);
+      } else {
+        impact.apply(visitor);
+        resolution.uncacheWorldImpact(element);
+      }
+    } else if (impactUse == DeferredLoadTask.IMPACT_USE) {
+      impact.apply(visitor);
+      // Impacts are uncached globally in [onImpactUsed].
+    } else if (impactUse == DumpInfoTask.IMPACT_USE) {
+      impact.apply(visitor);
+      dumpInfoTask.unregisterImpact(element);
+    } else {
+      impact.apply(visitor);
+    }
+  }
+
+  @override
+  void onImpactUsed(ImpactUseCase impactUse) {
+    if (impactUse == DeferredLoadTask.IMPACT_USE) {
+      resolution.emptyCache();
+    }
+  }
+}