Fix evaluation of constant map with duplicate keys.

tests/compiler/dart2js/constant_expression_evaluate_test.dart

Index: tests/compiler/dart2js/constant_expression_evaluate_test.dart
diff --git a/tests/compiler/dart2js/constant_expression_evaluate_test.dart b/tests/compiler/dart2js/constant_expression_evaluate_test.dart
index 49f3b51fe88eea9b2fc0992e85b1f2e8ab018350..9b5adcf2cc07e7d07e565910a0cf1e77cf497b07 100644
--- a/tests/compiler/dart2js/constant_expression_evaluate_test.dart
+++ b/tests/compiler/dart2js/constant_expression_evaluate_test.dart
@@ -79,6 +79,9 @@ const List<TestData> DATA = const [
         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)',