Add a diagnostic.getServerPort analysis server request.

pkg/analysis_server/lib/plugin/protocol/protocol.dart

Index: pkg/analysis_server/lib/plugin/protocol/protocol.dart
diff --git a/pkg/analysis_server/lib/plugin/protocol/protocol.dart b/pkg/analysis_server/lib/plugin/protocol/protocol.dart
index 49e19c16fbc0aa3b245e8664011fe8ad12398803..ea425cf2107d758549a8eb946e1262a1b1ca5ced 100644
--- a/pkg/analysis_server/lib/plugin/protocol/protocol.dart
+++ b/pkg/analysis_server/lib/plugin/protocol/protocol.dart
@@ -339,6 +339,14 @@ class Response {
       : _result = result;
 
   /**
+   * Create and return the `DEBUG_PORT_COULD_NOT_BE_OPENED` error response.
+   */
+  Response.debugPortCouldNotBeOpened(Request request, dynamic error)
+      : this(request.id,
+      error: new RequestError(
+          RequestErrorCode.DEBUG_PORT_COULD_NOT_BE_OPENED, '$error'));
+
+  /**
    * Initialize a newly created instance to represent the FILE_NOT_ANALYZED
    * error condition.
    */