Implement TypeMask.isDisjoint instead of using intersection.

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

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of types;
 
 class UnionTypeMask implements TypeMask {
   final Iterable<FlatTypeMask> disjointMasks;
 
   static const int MAX_UNION_LENGTH = 4;
@@ skipping 148 matching lines @@
         for (FlatTypeMask flatOther in other.disjointMasks) {
           intersections.add(current.intersection(flatOther, classWorld));
         }
       } else {
         intersections.add(current.intersection(other, classWorld));
       }
     }
     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);
     newList[0] = newList[0].nullable();
     return new UnionTypeMask._internal(newList);
   }
 
   TypeMask nonNullable() {
     if (!isNullable) return this;
     Iterable<FlatTypeMask> newIterable =
@@ skipping 179 matching lines @@
   int get hashCode {
     int hashCode = isNullable ? 86 : 43;
     // The order of the masks in [disjointMasks] must not affect the
     // hashCode.
     for (var mask in disjointMasks) {
       hashCode = (hashCode ^ mask.nonNullable().hashCode) & 0x3fffffff;
     }
     return hashCode;
   }
 }