Add ConstantConstructor to ConstantExpression system.

tests/compiler/dart2js/semantic_visitor_test.dart

 // Copyright (c) 2015, 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.semantics_visitor_test;
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 import 'package:compiler/src/constants/expressions.dart';
@@ skipping 2334 matching lines @@
         const [
           const Visit(VisitKind.VISIT_TOP_LEVEL_FUNCTION_DECL,
               element: 'function(m)',
               parameters: '(a,[b])',
               body: '{}'),
           const Visit(VisitKind.VISIT_REQUIRED_PARAMETER_DECL,
               element: 'parameter(m#a)',
               index: 0),
           const Visit(VisitKind.VISIT_OPTIONAL_PARAMETER_DECL,
               element: 'parameter(m#b)',
-              index: 1),
+              index: 1,
+              constant: 'null'),
         ]),
     const Test(
         '''
         m(a, [b = null]) {}
         ''',
         const [
           const Visit(VisitKind.VISIT_TOP_LEVEL_FUNCTION_DECL,
               element: 'function(m)',
               parameters: '(a,[b=null])',
               body: '{}'),
@@ skipping 28 matching lines @@
         ''',
         const [
           const Visit(VisitKind.VISIT_TOP_LEVEL_FUNCTION_DECL,
               element: 'function(m)',
               parameters: '(a,{b})',
               body: '{}'),
           const Visit(VisitKind.VISIT_REQUIRED_PARAMETER_DECL,
               element: 'parameter(m#a)',
               index: 0),
           const Visit(VisitKind.VISIT_NAMED_PARAMETER_DECL,
-              element: 'parameter(m#b)'),
+              element: 'parameter(m#b)',
+              constant: 'null'),
         ]),
     const Test(
         '''
         m(a, {b: null}) {}
         ''',
         const [
           const Visit(VisitKind.VISIT_TOP_LEVEL_FUNCTION_DECL,
               element: 'function(m)',
               parameters: '(a,{b: null})',
               body: '{}'),
@@ skipping 3788 matching lines @@
   VISIT_OPTIONAL_PARAMETER_DECL,
   VISIT_NAMED_PARAMETER_DECL,
   VISIT_REQUIRED_INITIALIZING_FORMAL_DECL,
   VISIT_OPTIONAL_INITIALIZING_FORMAL_DECL,
   VISIT_NAMED_INITIALIZING_FORMAL_DECL,
 
   ERROR_UNRESOLVED_POSTFIX,
 
   // TODO(johnniwinther): Add tests for more error cases.
 }