Gracefully degrade if no source for completion

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/src/analysis_server.dart';
 import 'package:analysis_server/src/channel/channel.dart';
@@ skipping 584 matching lines @@
   }
 
   @override
   Stream<SourcesChangedEvent> get onSourcesChanged => mockStream;
 
   @override
   TimestampedData<String> getContents(Source source) {
     return source.contents;
   }
 
+  @override
+  bool exists(Source source) {
+    return source != null && source.exists();
+  }
+
   noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
 }
 
 /**
  * Mock stream for tracking calls to listen and subscription.cancel.
  */
 class MockStream<E> implements Stream<E> {
   MockSubscription<E> mockSubscription = new MockSubscription<E>();
   int listenCount = 0;
 
@@ skipping 89 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);
   }
 }