Fix race condition with "server already started" error.

pkg/analysis_server/test/protocol_test.dart

Index: pkg/analysis_server/test/protocol_test.dart
diff --git a/pkg/analysis_server/test/protocol_test.dart b/pkg/analysis_server/test/protocol_test.dart
index 6597f1155926b44930ba59a7f001fba9d8332715..8d84eee478b25e5918dc6b246d1643dafc4bae48 100644
--- a/pkg/analysis_server/test/protocol_test.dart
+++ b/pkg/analysis_server/test/protocol_test.dart
@@ -38,6 +38,8 @@ main() {
     test('create_invalidRequest', RequestErrorTest.create_invalidRequest);
     test('create_internalError', RequestErrorTest.create_internalError);
     test('create_parseError', RequestErrorTest.create_parseError);
+    test('create_serverAlreadyStarted',
+        RequestErrorTest.create_serverAlreadyStarted);
     test('fromJson', RequestErrorTest.fromJson);
     test('toJson', RequestErrorTest.toJson);
   });
@@ -238,6 +240,12 @@ class RequestErrorTest {
     expect(error.message, "Internal error");
   }
 
+  static void create_serverAlreadyStarted() {
+    RequestError error = new RequestError.serverAlreadyStarted();
+    expect(error.code, RequestError.CODE_SERVER_ALREADY_STARTED);
+    expect(error.message, "Server already started");
+  }
+
   static void fromJson() {
     var json = {
         RequestError.CODE: RequestError.CODE_PARSE_ERROR,