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

pkg/compiler/lib/src/types/forwarding_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;
 
 /**
  * A type mask that wraps an other one, and delegate all its
  * implementation methods to it.
  */
@@ skipping 97 matching lines @@
       return forwardTo == other.forwardTo.nonNullable();
     }
   }
 
   bool operator==(other) {
     return equalsDisregardNull(other) && isNullable == other.isNullable;
   }
 
   int get hashCode => throw "Subclass should implement hashCode getter";
 }