Fix a timing-out test.

I forgot to update this to the new package server API, so it had a
future that was never completing.

Closes #1389
Closes #1390

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1674393002 .
diff --git a/test/must_pub_get_test.dart b/test/must_pub_get_test.dart
index 2233924..058ea89 100644
--- a/test/must_pub_get_test.dart
+++ b/test/must_pub_get_test.dart
@@ -269,7 +269,7 @@
       setUp(() {
         // Avoid using a path dependency because it triggers the full validation
         // logic. We want to be sure SDK-validation works without that logic.
-        servePackages((builder) {
+        globalPackageServer.add((builder) {
           builder.serve("foo", "3.0.0", pubspec: {
             "environment": {"sdk": ">=1.0.0 <2.0.0"}
           });