Ensure that failure to start an isolate all end up in the future.

tests/isolate/isolate_create_error_helper_echo.dart

Index: tests/isolate/isolate_create_error_helper_echo.dart
diff --git a/pkg/docgen/test/multi_library_code/lib/root_lib.dart b/tests/isolate/isolate_create_error_helper_echo.dart
similarity index 52%
copy from pkg/docgen/test/multi_library_code/lib/root_lib.dart
copy to tests/isolate/isolate_create_error_helper_echo.dart
index 74423abfeba22b68b7561d308758df0fdd7d9e60..b0231083c60a6d908a7e9b4ce92abbae3ff22347 100644
--- a/pkg/docgen/test/multi_library_code/lib/root_lib.dart
+++ b/tests/isolate/isolate_create_error_helper_echo.dart
@@ -2,12 +2,11 @@
 // 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 root_lib;
+/// Echo library reports its successful creation as an echo response.
+library isolate.create.error.helper.echo;
 
-export 'test_lib.dart';
-export 'test_lib_foo.dart';
-export 'test_lib_bar.dart';
+import "dart:isolate";
 
-class RootClass {
-  RootClass(int a, int b);
+void main(args, port) {
+  for (String s in args) port.send(s);
 }