Use entities in all masks.

pkg/compiler/lib/src/types/forwarding_type_mask.dart

Index: pkg/compiler/lib/src/types/forwarding_type_mask.dart
diff --git a/pkg/compiler/lib/src/types/forwarding_type_mask.dart b/pkg/compiler/lib/src/types/forwarding_type_mask.dart
index 73ac59d73b08f18d91981ae78c6c4bf43c26650e..04ee5c09d78aa39e27122282710ff65cf7286aad 100644
--- a/pkg/compiler/lib/src/types/forwarding_type_mask.dart
+++ b/pkg/compiler/lib/src/types/forwarding_type_mask.dart
@@ -54,15 +54,15 @@ abstract class ForwardingTypeMask implements TypeMask {
     return forwardTo.containsOnlyString(closedWorld);
   }
 
-  bool containsOnly(Entity cls) {
+  bool containsOnly(ClassEntity cls) {
     return forwardTo.containsOnly(cls);
   }
 
-  bool satisfies(Entity cls, ClosedWorld closedWorld) {
+  bool satisfies(ClassEntity cls, ClosedWorld closedWorld) {
     return forwardTo.satisfies(cls, closedWorld);
   }
 
-  bool contains(Entity cls, ClosedWorld closedWorld) {
+  bool contains(ClassEntity cls, ClosedWorld closedWorld) {
     return forwardTo.contains(cls, closedWorld);
   }
 
@@ -70,7 +70,7 @@ abstract class ForwardingTypeMask implements TypeMask {
     return forwardTo.containsAll(closedWorld);
   }
 
-  Entity singleClass(ClosedWorld closedWorld) {
+  ClassEntity singleClass(ClosedWorld closedWorld) {
     return forwardTo.singleClass(closedWorld);
   }