Provide and respect HTTP cache headers in pub serve.

test/serve/utils.dart

Index: test/serve/utils.dart
diff --git a/test/serve/utils.dart b/test/serve/utils.dart
index bf760226b1f113ae899c3f1d54b92badd7e73a3e..9f95446353a64296777f9db7132a65bc2f3820bc 100644
--- a/test/serve/utils.dart
+++ b/test/serve/utils.dart
@@ -231,9 +231,10 @@ void endPubServe() {
 /// invokes [callback] with the response.
 ///
 /// [root] indicates which server should be accessed, and defaults to "web".
-Future<http.Response> scheduleRequest(String urlPath, {String root}) {
+Future<http.Response> scheduleRequest(String urlPath,
+    {String root, Map<String, String> headers}) {
   return schedule(() {
-    return http.get(_getServerUrlSync(root, urlPath));
+    return http.get(_getServerUrlSync(root, urlPath), headers: headers);
   }, "request $urlPath");
 }