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

sdk/lib/_internal/pub/lib/src/command/serve.dart

Index: sdk/lib/_internal/pub/lib/src/command/serve.dart
diff --git a/sdk/lib/_internal/pub/lib/src/command/serve.dart b/sdk/lib/_internal/pub/lib/src/command/serve.dart
index d7e9de5f56f7724bc9cbedec5b36de47d0fa7bbb..e43a9fc2bfa8f6c67b382bf74165e3f3f0d242f6 100644
--- a/sdk/lib/_internal/pub/lib/src/command/serve.dart
+++ b/sdk/lib/_internal/pub/lib/src/command/serve.dart
@@ -98,15 +98,17 @@ class ServeCommand extends BarbackCommand {
     var directoryLength = sourceDirectories.map((dir) => dir.length)
         .reduce(math.max);
 
-    var server = await environment.startAdminServer(adminPort);
-    server.results.listen((_) {
-      // The admin server produces no result values.
-      assert(false);
-    }, onError: _fatalError);
-
-    if (logAdminUrl) {
-      log.message("Running admin server on "
-          "${log.bold('http://$hostname:${server.port}')}");
+    if (adminPort != null) {
+      var server = await environment.startAdminServer(adminPort);
+      server.results.listen((_) {
+        // The admin server produces no result values.
+        assert(false);
+      }, onError: _fatalError);
+
+      if (logAdminUrl) {
+        log.message("Running admin server on "
+                    "${log.bold('http://$hostname:${server.port}')}");
+      }
     }
 
     // Start up the servers. We pause updates while this is happening so