pub: don't start the admin server if no --admin-port flag was given.

sdk/lib/_internal/pub/test/serve/utils.dart

Index: sdk/lib/_internal/pub/test/serve/utils.dart
diff --git a/sdk/lib/_internal/pub/test/serve/utils.dart b/sdk/lib/_internal/pub/test/serve/utils.dart
index f84eb9a353f97c13dc97df922401882306a2451d..be87afc46d2acd87adf66c56bfbafd8963bf2fbb 100644
--- a/sdk/lib/_internal/pub/test/serve/utils.dart
+++ b/sdk/lib/_internal/pub/test/serve/utils.dart
@@ -155,6 +155,7 @@ ScheduledProcess startPubServe({Iterable<String> args,
     "serve",
     "--port=0", // Use port 0 to get an ephemeral port.
     "--force-poll",
+    "--admin-port=0", // Use port 0 to get an ephemeral port.
     "--log-admin-url"
   ];
 
@@ -223,6 +224,7 @@ final _parsePortRegExp = new RegExp(r"([^ ]+) +on http://localhost:(\d+)");
 /// Parses the port number from the "Running admin server on localhost:1234"
 /// line printed by pub serve.
 bool _parseAdminPort(String line) {
+  expect(line, startsWith('Running admin server on'));
   var match = _parsePortRegExp.firstMatch(line);
   if (match == null) return false;
   _adminPort = int.parse(match[2]);