All strong mode cleaning of dart2js.

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 04ee5c09d78aa39e27122282710ff65cf7286aad..974f255195aedd33b2f33b809cb007e2c59271d4 100644
--- a/pkg/compiler/lib/src/types/forwarding_type_mask.dart
+++ b/pkg/compiler/lib/src/types/forwarding_type_mask.dart
@@ -93,7 +93,8 @@ abstract class ForwardingTypeMask implements TypeMask {
     return forwardTo.intersection(other, closedWorld);
   }
 
-  bool needsNoSuchMethodHandling(Selector selector, ClosedWorld closedWorld) {
+  bool needsNoSuchMethodHandling(
+      Selector selector, covariant ClosedWorld closedWorld) {
     return forwardTo.needsNoSuchMethodHandling(selector, closedWorld);
   }