dart2js: Do not use JSDouble for native behavior return types.

tests/compiler/dart2js/cpa_inference_test.dart

 // Copyright (c) 2012, 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.
 
 import 'dart:async';
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 import 'package:compiler/src/types/types.dart';
 import 'package:compiler/src/inferrer/concrete_types_inferrer.dart';
 import 'package:compiler/src/universe/universe.dart' show Selector;
@@ skipping 1436 matching lines @@
     result.checkNodeHasUnknownType('b');
     final expectedCType = [result.growableList];
     result.checkNodeHasType('c', expectedCType);
     result.checkNodeHasType('cNull', maybe(expectedCType));
     final expectedDType = [result.string];
     result.checkNodeHasType('d', expectedDType);
     result.checkNodeHasType('dNull', maybe(expectedDType));
     final expectedEType = [result.int];
     result.checkNodeHasType('e', expectedEType);
     result.checkNodeHasType('eNull', maybe(expectedEType));
-    final expectedFType = [result.double];
+    final expectedFType = [result.num];
     result.checkNodeHasType('f', expectedFType);
     result.checkNodeHasType('fNull', maybe(expectedFType));
     final expectedGType = [result.num];
     result.checkNodeHasType('g', expectedGType);
     result.checkNodeHasType('gNull', maybe(expectedGType));
     final expectedType = [result.bool];
     result.checkNodeHasType('h', expectedType);
     result.checkNodeHasType('hNull', maybe(expectedType));
     final expectedIType = [result.base('B'),
                            result.base('BB'),
@@ skipping 677 matching lines @@
     testClosures10,
     testClosures11,
     testClosures12,
     testRefinement,
     testDefaultArguments,
     testSuperConstructorCall,
     testSuperConstructorCall2,
     testSuperConstructorCall3,
   ], (f) => f()));
 }