Fix compiler crash when we declare several (global) variables, but only define some of them.

tests/compiler/dart2js/constant_expression_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 constant_expression_test;
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 import 'package:compiler/src/constants/expressions.dart';
@@ skipping 129 matching lines @@
             }),
       ]),
   const TestData(
       '''
 class A<T> implements B {
   final field1;
   const A({this.field1:42});
 }
 class B<S> implements C {
   const factory B({field1}) = A<B<S>>;
-  // TODO(johnniwinther): Enable this when the constructor evaluator doesn't 
-  // crash:
   const factory B.named() = A<S>;
 }
 class C<U> {
   const factory C({field1}) = A<B<double>>;
 }
 ''',
       const [
         const ConstantData('const A()', ConstantExpressionKind.CONSTRUCTED,
             type: 'A<dynamic>', fields: const {'field(A#field1)': '42'}),
         const ConstantData(
@@ skipping 83 matching lines @@
         String expected = data.fields[name];
         Expect.equals(
             expected,
             expression,
             "Unexpected field expression ${expression} for field '$name' in "
             "contant `${constant.toDartText()}`, expected '${expected}'.");
       });
     }
   });
 }