Forget elements from resolution also.

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 baecb6d4af4cb8e038b12c8bded5ace0244d38a7..68089a7f1aca6c438c665cc16528012d5c61cddc 100644
--- a/pkg/compiler/lib/src/compiler.dart
+++ b/pkg/compiler/lib/src/compiler.dart
@@ -1403,6 +1403,7 @@ abstract class Compiler {
   }
 
   void forgetElement(Element element) {
+    resolution.forgetElement(element);
     enqueuer.forgetElement(element);
     if (element is MemberElement) {
       for (Element closure in element.nestedClosures) {
@@ -2010,6 +2011,11 @@ class _CompilerResolution implements Resolution {
   bool hasBeenResolved(Element element) {
     return _worldImpactCache.containsKey(element);
   }
+
+  @override
+  void forgetElement(Element element) {
+    _worldImpactCache.remove(element);
+  }
 }
 
 // TODO(johnniwinther): Move [ParserTask], [PatchParserTask], [DietParserTask]