Implement TypeMask.isDisjoint instead of using intersection.

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 ce29f506290fa1d4cf1797f8b3068ef707002bc4..a4400c616ad6a2af51069b7415fa89358fde34d2 100644
--- a/pkg/compiler/lib/src/types/forwarding_type_mask.dart
+++ b/pkg/compiler/lib/src/types/forwarding_type_mask.dart
@@ -84,6 +84,10 @@ abstract class ForwardingTypeMask implements TypeMask {
     return forwardTo.union(other, classWorld);
   }
 
+  bool isDisjoint(TypeMask other, ClassWorld classWorld) {
+    return forwardTo.isDisjoint(other, classWorld);
+  }
+
   TypeMask intersection(TypeMask other, ClassWorld classWorld) {
     return forwardTo.intersection(other, classWorld);
   }