Add error codes to server protocol (#25958).

pkg/analysis_server/test/protocol_server_test.dart

 // Copyright (c) 2014, 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.
 
 library test.computer.element;
 
 import 'dart:mirrors';
 
 import 'package:analysis_server/src/constants.dart';
 import 'package:analysis_server/src/protocol_server.dart';
@@ skipping 51 matching lines @@
       TYPE: 'COMPILE_TIME_ERROR',
       LOCATION: {
         FILE: 'foo.dart',
         OFFSET: 10,
         LENGTH: 20,
         START_LINE: 3,
         START_COLUMN: 2
       },
       MESSAGE: 'my message',
       CORRECTION: 'my correction',
+      CODE: 'ambiguous_export',
       HAS_FIX: false
     });
   }
 
   void test_fromEngine_noCorrection() {
     when(engineError.correction).thenReturn(null);
     AnalysisError error = newAnalysisError_fromEngine(lineInfo, engineError);
     expect(error.toJson(), {
       SEVERITY: 'ERROR',
       TYPE: 'COMPILE_TIME_ERROR',
       LOCATION: {
         FILE: 'foo.dart',
         OFFSET: 10,
         LENGTH: 20,
         START_LINE: 3,
         START_COLUMN: 2
       },
       MESSAGE: 'my message',
+      CODE: 'ambiguous_export',
       HAS_FIX: false
     });
   }
 
   void test_fromEngine_noLineInfo() {
     when(engineError.correction).thenReturn(null);
     AnalysisError error = newAnalysisError_fromEngine(null, engineError);
     expect(error.toJson(), {
       SEVERITY: 'ERROR',
       TYPE: 'COMPILE_TIME_ERROR',
       LOCATION: {
         FILE: 'foo.dart',
         OFFSET: 10,
         LENGTH: 20,
         START_LINE: -1,
         START_COLUMN: -1
       },
       MESSAGE: 'my message',
+      CODE: 'ambiguous_export',
       HAS_FIX: false
     });
   }
 }
 
 @reflectiveTest
 class EnumTest {
   void test_AnalysisErrorSeverity() {
     new EnumTester<engine.ErrorSeverity, AnalysisErrorSeverity>().run(
         (engine.ErrorSeverity engineErrorSeverity) =>
@@ skipping 64 matching lines @@
           ApiEnum apiValue = convert(engineValue);
           expect(apiValue, equals(expectedResult));
         }
       } else {
         ApiEnum apiValue = convert(engineValue);
         expect(apiValue.name, equals(enumName));
       }
     });
   }
 }