Don't expect String on read-errors.

tests/compiler/dart2js/missing_file_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.
 
 // Test that the compiler can handle imports when package root has not been set.
 
 library dart2js.test.missing_file;
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
@@ skipping 51 matching lines @@
   asyncTest(() => compiler.run(main).then((_) {
     Expect.equals(1, errors.length);
     Expect.equals(expectedMessage,
                   errors[0]);
   }));
 }
 
 void main() {
   runCompiler(Uri.parse('memory:main.dart'),
               "Error: Can't read 'memory:foo.dart' "
-              "(No such file memory:foo.dart).");
+              "(Exception: No such file memory:foo.dart).");
   runCompiler(Uri.parse('memory:foo.dart'),
               "Error: Can't read 'memory:foo.dart' "
-              "(No such file memory:foo.dart).");
+              "(Exception: No such file memory:foo.dart).");
   runCompiler(Uri.parse('dart:foo'),
               'Error: Library not found "dart:foo".');
 }