Fix "pub serve" for VM tests.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1095693002

CHANGELOG.md

+### 0.12.0-beta.9
+
+* Fix running VM tests against `pub serve`.
+
 ### 0.12.0-beta.8
 
 * Add support for configuring timeouts on a test, group, and suite basis. Test

lib/pub_serve.dart

@@ -29,6 +29,7 @@ class PubServeTransformer extends Transformer implements DeclaringTransformer {
 
     transform.addOutput(
         new Asset.fromString(id.addExtension('.vm_test.dart'), '''
+import "package:test/src/backend/metadata.dart";
 import "package:test/src/runner/vm/isolate_listener.dart";
 
 import "${p.url.basename(id.path)}" as test;