Remove WorldImpact from caches when no longer needed.

pkg/compiler/lib/src/compiler.dart

Index: pkg/compiler/lib/src/compiler.dart
diff --git a/pkg/compiler/lib/src/compiler.dart b/pkg/compiler/lib/src/compiler.dart
index 392b2026b6fc8cbf6e8d66de3db1a9ca40e7f80d..43c0d44dd48214b906e0198d385989808ea9d0b6 100644
--- a/pkg/compiler/lib/src/compiler.dart
+++ b/pkg/compiler/lib/src/compiler.dart
@@ -137,6 +137,7 @@ import 'universe/universe.dart' show
 import 'universe/use.dart' show
     StaticUse;
 import 'universe/world_impact.dart' show
+    ImpactStrategy,
     WorldImpact;
 import 'util/util.dart' show
     Link,
@@ -157,6 +158,8 @@ abstract class Compiler {
 
   final CacheStrategy cacheStrategy;
 
+  ImpactStrategy impactStrategy = const ImpactStrategy();
+
   /**
    * Map from token to the first preceding comment token.
    */
@@ -975,6 +978,9 @@ abstract class Compiler {
     // something to the resolution queue.  So we cannot wait with
     // this until after the resolution queue is processed.
     deferredLoadTask.beforeResolution(this);
+    impactStrategy = backend.createImpactStrategy(
+        supportDeferredLoad: deferredLoadTask.isProgramSplit,
+        supportDumpInfo: dumpInfo);
 
     phase = PHASE_RESOLVING;
     if (analyzeAll) {
@@ -1130,6 +1136,9 @@ abstract class Compiler {
     }
     emptyQueue(world);
     world.queueIsClosed = true;
+    // Notify the impact strategy impacts are no longer needed for this
+    // enqueuer.
+    impactStrategy.onImpactUsed(world.impactUse);
     backend.onQueueClosed();
     assert(compilationFailed || world.checkNoEnqueuedInvokedInstanceMethods());
   }
@@ -2019,6 +2028,20 @@ class _CompilerResolution implements Resolution {
   }
 
   @override
+  void uncacheWorldImpact(Element element) {
+    assert(invariant(element, _worldImpactCache[element] != null,
+        message: "WorldImpact not computed for $element."));
+    _worldImpactCache[element] = const WorldImpact();
+  }
+
+  @override
+  void emptyCache() {
+    for (Element element in _worldImpactCache.keys) {
+      _worldImpactCache[element] = const WorldImpact();
+    }
+  }
+
+  @override
   bool hasBeenResolved(Element element) {
     return _worldImpactCache.containsKey(element);
   }