Don't insert additional ':' when changing a named argument name.

pkg/analysis_server/test/domain_completion_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.domain.completion;
 
 import 'dart:async';
 
 import 'package:analysis_server/plugin/protocol/protocol.dart';
 import 'package:analysis_server/src/domain_completion.dart';
@@ skipping 33 matching lines @@
     assertHasResult(CompletionSuggestionKind.NAMED_ARGUMENT, 'one: ',
         relevance: DART_RELEVANCE_NAMED_PARAMETER);
     assertHasResult(CompletionSuggestionKind.NAMED_ARGUMENT, 'two: ',
         relevance: DART_RELEVANCE_NAMED_PARAMETER);
     expect(suggestions, hasLength(2));
   }
 
   test_ArgumentList_imported_function_named_param_label1() async {
     addTestFile('main() { int.parse("16", r^: 16);}');
     await getSuggestions();
-    assertHasResult(CompletionSuggestionKind.NAMED_ARGUMENT, 'radix: ',
+    assertHasResult(CompletionSuggestionKind.NAMED_ARGUMENT, 'radix',
         relevance: DART_RELEVANCE_NAMED_PARAMETER);
-    assertHasResult(CompletionSuggestionKind.NAMED_ARGUMENT, 'onError: ',
+    assertHasResult(CompletionSuggestionKind.NAMED_ARGUMENT, 'onError',
         relevance: DART_RELEVANCE_NAMED_PARAMETER);
     expect(suggestions, hasLength(2));
   }
 
   test_ArgumentList_imported_function_named_param_label3() async {
     addTestFile('main() { int.parse("16", ^: 16);}');
     await getSuggestions();
     assertHasResult(CompletionSuggestionKind.NAMED_ARGUMENT, 'radix: ',
         relevance: DART_RELEVANCE_NAMED_PARAMETER);
     assertHasResult(CompletionSuggestionKind.NAMED_ARGUMENT, 'onError: ',
@@ skipping 539 matching lines @@
 }
     ''');
     await waitForTasksFinished();
     Request request =
         new CompletionGetSuggestionsParams(testFile, 0).toRequest('0');
     Response response = handler.handleRequest(request);
     expect(response.error, isNotNull);
     expect(response.error.code, RequestErrorCode.NO_INDEX_GENERATED);
   }
 }