Implement TypeMask.isDisjoint instead of using intersection.

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

Index: pkg/compiler/lib/src/types/type_mask.dart
diff --git a/pkg/compiler/lib/src/types/type_mask.dart b/pkg/compiler/lib/src/types/type_mask.dart
index 3339738fa875298cccc42739cb124a1bfc495b94..736df0d614f3ef32e2e76d995e4648cb5fdbc9a2 100644
--- a/pkg/compiler/lib/src/types/type_mask.dart
+++ b/pkg/compiler/lib/src/types/type_mask.dart
@@ -329,6 +329,10 @@ abstract class TypeMask implements ReceiverConstraint, AbstractValue {
    */
   TypeMask union(TypeMask other, ClassWorld classWorld);
 
+
+  /// Returns whether the intersection of this and [other] is empty.
+  bool isDisjoint(TypeMask other, ClassWorld classWorld);
+
   /**
    * Returns a type mask representing the intersection of [this] and [other].
    */