dart2js cps: Fix return type for native functions.

pkg/compiler/lib/src/cps_ir/cps_ir_nodes.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.
 library dart2js.ir_nodes;
 
 import 'dart:collection';
 import 'cps_fragment.dart' show CpsFragment;
 import '../constants/values.dart' as values;
 import '../dart_types.dart' show DartType, InterfaceType, TypeVariableType;
 import '../elements/elements.dart';
@@ skipping 1511 matching lines @@
   bool get isSafeForElimination => true;
   bool get isSafeForReordering => true;
 
   void setParentPointers() {
     _setParentsOnList(arguments, this);
   }
 }
 
 class ForeignCode extends UnsafePrimitive {
   final js.Template codeTemplate;
-  final TypeMask type;
+  final TypeMask storedType;

[asgerf, 2016/01/19 19:22:04]

This field clashed with the .type field inherited from Primitive. But since this
field is final any attempts to reassign it (in type propagation) would instead
change the inherited (un-readable) field from Primitive.

I will open an analyzer feature request to detect this kind of nonsense.

Apart from renaming the field, another solution would be to just never change
its type after initialization (which is essentially what happened before, but in
a very subtle way).

   final List<Reference<Primitive>> arguments;
   final native.NativeBehavior nativeBehavior;
   final FunctionElement dependency;
 
-  ForeignCode(this.codeTemplate, this.type, List<Primitive> arguments,
+  ForeignCode(this.codeTemplate, this.storedType, List<Primitive> arguments,
       this.nativeBehavior, {this.dependency})
       : this.arguments = _referenceList(arguments);
 
   accept(Visitor visitor) => visitor.visitForeignCode(this);
 
   bool get hasValue => true;
 
   void setParentPointers() {
     _setParentsOnList(arguments, this);
   }
@@ skipping 1282 matching lines @@
     plug(new Branch.loose(_definitions.getCopy(node.condition),
         _copies[node.trueContinuation.definition],
         _copies[node.falseContinuation.definition])
       ..isStrictCheck = node.isStrictCheck);
   }
 
   visitUnreachable(Unreachable node) {
     plug(new Unreachable());
   }
 }