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

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

 // Copyright (c) 2014, 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 ValueTypeMask extends ForwardingTypeMask {
   final TypeMask forwardTo;
   final value;
 
@@ skipping 29 matching lines @@
   bool operator==(other) => super == other;
 
   int get hashCode {
     return computeHashCode(value, isNullable, forwardTo);
   }
 
   String toString() {
     return 'Value mask: [$value] type: $forwardTo';
   }
 }