Add a diagnostic.getServerPort analysis server request.

pkg/analysis_server/lib/src/server/diagnostic_server.dart

Index: pkg/analysis_server/lib/src/server/diagnostic_server.dart
diff --git a/pkg/analysis_server/lib/src/server/diagnostic_server.dart b/pkg/analysis_server/lib/src/server/diagnostic_server.dart
new file mode 100644
index 0000000000000000000000000000000000000000..b147a43ed55266e1460d6449d41acd761ebafdba
--- /dev/null
+++ b/pkg/analysis_server/lib/src/server/diagnostic_server.dart
@@ -0,0 +1,16 @@
+// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:async';
+
+/**
+ * A handle to start, and return the current port of, a diagnostic server.
+ */
+abstract class DiagnosticServer {
+  /**
+   * Return the port of the diagnostic web server. If the server is not running
+   * this call will start the server.
+   */
+  Future<int> getServerPort();
+}