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