Don't expect String on read-errors.

tests/compiler/dart2js/missing_file_test.dart

Index: tests/compiler/dart2js/missing_file_test.dart
diff --git a/tests/compiler/dart2js/missing_file_test.dart b/tests/compiler/dart2js/missing_file_test.dart
index 349b61cb6306406e6f669ea68f6b621c57ff73d4..06b75dd8798d450c0a91233ff7ebf282b90a8645 100644
--- a/tests/compiler/dart2js/missing_file_test.dart
+++ b/tests/compiler/dart2js/missing_file_test.dart
@@ -69,10 +69,10 @@ void runCompiler(Uri main, String expectedMessage) {
 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".');
 }