Remove WorldImpact from caches when no longer needed.

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

Index: pkg/compiler/lib/src/common/codegen.dart
diff --git a/pkg/compiler/lib/src/common/codegen.dart b/pkg/compiler/lib/src/common/codegen.dart
index 37c9855a3f4c3aed788848be2f579e5cdffba8e1..7e9b40fa95347261885d831d8ba543b3ab55f124 100644
--- a/pkg/compiler/lib/src/common/codegen.dart
+++ b/pkg/compiler/lib/src/common/codegen.dart
@@ -28,7 +28,8 @@ import '../universe/use.dart' show
     TypeUse;
 import '../universe/world_impact.dart' show
     WorldImpact,
-    WorldImpactBuilder;
+    WorldImpactBuilder,
+    WorldImpactVisitor;
 import '../util/util.dart' show
     Pair,
     Setlet;
@@ -78,6 +79,12 @@ class _CodegenImpact extends WorldImpactBuilder implements CodegenImpact {
 
   _CodegenImpact(this.registry);
 
+  void apply(WorldImpactVisitor visitor) {
+    staticUses.forEach(visitor.visitStaticUse);
+    dynamicUses.forEach(visitor.visitDynamicUse);
+    typeUses.forEach(visitor.visitTypeUse);
+  }
+
   void registerCompileTimeConstant(ConstantValue constant) {
     if (_compileTimeConstants == null) {
       _compileTimeConstants = new Setlet<ConstantValue>();
@@ -262,8 +269,6 @@ class CodegenWorkItem extends WorkItem {
     if (world.isProcessed(element)) return const WorldImpact();
 
     registry = new CodegenRegistry(compiler, element);
-    var impact = compiler.codegen(this, world);
-    compiler.dumpInfoTask.registerImpact(element, impact);
-    return impact;
+    return compiler.codegen(this, world);
   }
 }