Fix strong mode error in test.

test/parser_test.dart

 // Copyright (c) 2013, 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 test.parser_test;
 
 import 'dart:convert';
 import 'package:test/test.dart';
 import 'package:source_maps/source_maps.dart';
 import 'package:source_span/source_span.dart';
@@ skipping 232 matching lines @@
       MAP_WITH_SOURCE_LOCATION,
       MAP_WITH_SOURCE_LOCATION_AND_NAME
     ]) {
       var mapping = parseJson(expected);
       expect(mapping.toJson(), equals(expected));
 
       mapping = parseJsonExtended(expected);
       expect(mapping.toJson(), equals(expected));
     }
     // Invalid for this case
-    expect(() => parseJson(SOURCE_MAP_BUNDLE), throws);
+    expect(() => parseJson(SOURCE_MAP_BUNDLE as dynamic), throws);
 
     var mapping = parseJsonExtended(SOURCE_MAP_BUNDLE);
     expect(mapping.toJson(), equals(SOURCE_MAP_BUNDLE));
   });
 }