Further cleanup of element use in type_graph_nodes

pkg/compiler/lib/src/types/map_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 masks;
 
 /**
  * A [MapTypeMask] is a [TypeMask] for a specific allocation
  * site of a map (currently only internal Map class) that will get specialized
  * once the [TypeGraphInferrer] phase finds a key and/or value type for it.
  */
 class MapTypeMask extends ForwardingTypeMask {
   final TypeMask forwardTo;
 
   // The [Node] where this type mask was created.
   final Node allocationNode;
 
-  // The [Entity] where this type mask was created.
-  final Entity allocationElement;
+  // The [MemberEntity] where this type mask was created.
+  final MemberEntity allocationElement;
 
   // The value type of this map.
   final TypeMask valueType;
 
   // The key type of this map.
   final TypeMask keyType;
 
   MapTypeMask(this.forwardTo, this.allocationNode, this.allocationElement,
       this.keyType, this.valueType);
 
@@ skipping 74 matching lines @@
 
   int get hashCode {
     return computeHashCode(
         allocationNode, isNullable, keyType, valueType, forwardTo);
   }
 
   String toString() {
     return 'Map mask: [$keyType/$valueType] type: $forwardTo';
   }
 }