Add ElementKind for factory constructors.

tests/compiler/dart2js/js_backend_cps_ir_constructor_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.
 
 // Tests of interceptors.
 
 library constructor_test;
 
 import 'js_backend_cps_ir.dart';
 
@@ skipping 198 matching lines @@
   A.b(this.x);
 }
 main() {
   print(new A().x);
 }""", r"""
 function() {
   return new V.A(1);
 }"""),
 
 
-const TestEntry.forMethod('function(Foo#make)', r"""
+const TestEntry.forMethod('factory_constructor(Foo#make)', r"""
 class Foo {
   factory Foo.make(x) {
     print('Foo');
     return new Foo.create(x);
   }
   var x;
   Foo.create(this.x);
 }
 main() {
   print(new Foo.make(5));
@@ skipping 29 matching lines @@
   new A(5).typevar;
 }""", r"""
 function() {
   V.B$(5, P.$int).get$typevar();
 }"""),
 ];
 
 void main() {
   runTests(tests);
 }