Remove WorldImpact from caches when no longer needed.

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

Index: pkg/compiler/lib/src/common/resolution.dart
diff --git a/pkg/compiler/lib/src/common/resolution.dart b/pkg/compiler/lib/src/common/resolution.dart
index 727f3aa8632f19128a7d4b16302b1c02ff4dd192..952a1f413c051d973b39301639e802a97ae831ba 100644
--- a/pkg/compiler/lib/src/common/resolution.dart
+++ b/pkg/compiler/lib/src/common/resolution.dart
@@ -186,8 +186,16 @@ abstract class Resolution {
   DartType resolveTypeAnnotation(Element element, TypeAnnotation node);
 
   bool hasBeenResolved(Element element);
+
+  /// Returns the precomputed [WorldImpact] for [element].
   WorldImpact getWorldImpact(Element element);
+
+  /// Computes the [WorldImpact] for [element].
   WorldImpact computeWorldImpact(Element element);
+
+  /// Removes the [WorldImpact] for [element] from the resolution cache. Later
+  /// calls to [getWorldImpact] or [computeWorldImpact] returns an empty impact.
+  void uncacheWorldImpact(Element element);
 }
 
 // TODO(johnniwinther): Rename to `Parser` or `ParsingContext`.