Move dart2js from sdk/lib/_internal/compiler to pkg/compiler

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 327 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;
   }
 }