Add admin server that supports web socket API.

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

Index: sdk/lib/_internal/pub/lib/src/barback/build_environment.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/build_environment.dart b/sdk/lib/_internal/pub/lib/src/barback/build_environment.dart
index 99890b724788e02a8e018a91e9245bfd97d0374a..f155374d7bf82e536e4d270da6a1223cda9cdfea 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/build_environment.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/build_environment.dart
@@ -17,12 +17,13 @@ import '../io.dart';
 import '../log.dart' as log;
 import '../package.dart';
 import '../package_graph.dart';
+import 'admin_server.dart';
 import 'build_directory.dart';
 import 'dart_forwarding_transformer.dart';
 import 'dart2js_transformer.dart';
 import 'load_all_transformers.dart';
 import 'pub_package_provider.dart';
-import 'server.dart';
+import 'barback_server.dart';
 
 /// The entire "visible" state of the assets of a package and all of its
 /// dependencies, taking into account the user's configuration when running pub.
@@ -67,6 +68,9 @@ class BuildEnvironment {
     });
   }
 
+  /// The server for the Web Socket API and admin interface.
+  AdminServer _adminServer;
+
   /// The public directories in the root package that are available for
   /// building, keyed by their root directory.
   final _directories = new Map<String, BuildDirectory>();
@@ -132,6 +136,21 @@ class BuildEnvironment {
     return _builtInTransformers;
   }
 
+  /// 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() {
+    // Can only start once.
+    assert(_adminServer == null);
+
+    // The admin server is bound to one before the base port, unless it's
+    // ephemeral in which case the admin port is too.
+    var port = _basePort == 0 ? 0 : _basePort - 1;
+
+    return AdminServer.bind(this, _hostname, port)
+        .then((server) => _adminServer = server);
+  }
+
   /// Binds a new port to serve assets from within [rootDirectory] in the
   /// entrypoint package.
   ///