Fix race condition with "server already started" error.

pkg/analysis_server/lib/src/protocol.dart

Index: pkg/analysis_server/lib/src/protocol.dart
diff --git a/pkg/analysis_server/lib/src/protocol.dart b/pkg/analysis_server/lib/src/protocol.dart
index 4b5a561900752f9e6fde93842254ea3d12dab3cf..c219cf3a267812228b55d20bbc22881b60c1373a 100644
--- a/pkg/analysis_server/lib/src/protocol.dart
+++ b/pkg/analysis_server/lib/src/protocol.dart
@@ -353,6 +353,12 @@ class RequestError {
    * server. An error occurred on the server while parsing the JSON text.
    */
   static const int CODE_PARSE_ERROR = -32700;
+  
+  /**
+   * An error code indicating that the analysis server has already been
+   * started (and hence won't accept new connections).
+   */
+  static const int CODE_SERVER_ALREADY_STARTED = -32701;
 
   /**
    * An error code indicating an invalid request. The JSON sent is not a valid
@@ -414,6 +420,13 @@ class RequestError {
   RequestError.parseError() : this(CODE_PARSE_ERROR, "Parse error");
 
   /**
+   * Initialize a newly created [Error] to indicate that the analysis server
+   * has already been started (and hence won't accept new connections).
+   */
+  RequestError.serverAlreadyStarted()
+    : this(CODE_SERVER_ALREADY_STARTED, "Server already started");
+  
+  /**
    * Initialize a newly created [Error] to indicate an invalid request. The
    * JSON sent is not a valid [Request] object.
    */