Implement TypeMask.isDisjoint instead of using intersection.

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

Index: pkg/compiler/lib/src/types/union_type_mask.dart
diff --git a/pkg/compiler/lib/src/types/union_type_mask.dart b/pkg/compiler/lib/src/types/union_type_mask.dart
index 4a8397e3501b8be1ffa76bca02cbed2a9a57d0b2..24a019144a87e3d76477db32517f90d80fc4f17a 100644
--- a/pkg/compiler/lib/src/types/union_type_mask.dart
+++ b/pkg/compiler/lib/src/types/union_type_mask.dart
@@ -166,6 +166,13 @@ class UnionTypeMask implements TypeMask {
     return new TypeMask.unionOf(intersections, classWorld);
   }
 
+  bool isDisjoint(TypeMask other, ClassWorld classWorld) {
+    for (var current in disjointMasks) {
+      if (!current.isDisjoint(other, classWorld)) return false;
+    }
+    return true;
+  }
+
   TypeMask nullable() {
     if (isNullable) return this;
     List<FlatTypeMask> newList = new List<FlatTypeMask>.from(disjointMasks);