Fix evaluation of constant map with duplicate keys.

tests/compiler/dart2js/constant_expression_evaluate_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.constants.expressions.evaluate_test;
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 import 'package:compiler/src/constants/evaluation.dart';
@@ skipping 61 matching lines @@
     const ConstantData('const <int>[0, 1]', const {
         const {} : 'ListConstant(<int>[IntConstant(0), IntConstant(1)])' }),
     const ConstantData('const {0: 1, 2: 3}',
         const { const {} :
           'MapConstant({IntConstant(0): IntConstant(1), '
                        'IntConstant(2): IntConstant(3)})' }),
     const ConstantData('const <int, int>{0: 1, 2: 3}',
         const { const {} :
           'MapConstant(<int, int>{IntConstant(0): IntConstant(1), '
                                  'IntConstant(2): IntConstant(3)})' }),
+    const ConstantData('const <int, int>{0: 1, 0: 2}',
+        const { const {} :
+        'MapConstant(<int, int>{IntConstant(0): IntConstant(2)})' }),
     const ConstantData(
         'const bool.fromEnvironment("foo", defaultValue: false)',
         const { const {} : 'BoolConstant(false)',
                 const {'foo': 'true'} : 'BoolConstant(true)'}),
     const ConstantData(
         'const int.fromEnvironment("foo", defaultValue: 42)',
         const { const {} : 'IntConstant(42)',
                 const {'foo': '87'} : 'IntConstant(87)'}),
     const ConstantData(
         'const String.fromEnvironment("foo", defaultValue: "bar")',
@@ skipping 145 matching lines @@
       Environment environment = new MemoryEnvironment(compiler, env);
       ConstantValue value =
           constant.evaluate(environment, DART_CONSTANT_SYSTEM);
       String valueText = value.toStructuredText();
       Expect.equals(expectedText, valueText,
           "Unexpected value '${valueText}' for contant "
           "`${constant.toDartText()}`, expected '${expectedText}'.");
     });
   });
 }