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

sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart

Index: sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart b/sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart
index a8a56f0df048f8eb2b6c84f1ae24643a9ddb241e..bfd97b35aff834ff353ff2fe77ff0d46f067d7b0 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart
@@ -184,10 +184,14 @@ class AssetEnvironment {
   /// Starts up the admin server on an appropriate port and returns it.
   ///
   /// This may only be called once on the build environment.
-  Future<AdminServer> startAdminServer(int port) {
+  Future<AdminServer> startAdminServer([int port]) {
     // Can only start once.
     assert(_adminServer == null);
 
+    // The admin server is bound to one before the base port by default, unless
+    // it's ephemeral in which case the admin port is too.
+    if (port == null) port = _basePort == 0 ? 0 : _basePort - 1;
+
     return AdminServer.bind(this, _hostname, port)
         .then((server) => _adminServer = server);
   }