suggest after import prefix in part files - fixes #25023

pkg/analysis_server/test/services/completion/dart/completion_contributor_util.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.services.completion.dart.util;
 
 import 'dart:async';
 
 import 'package:analysis_server/plugin/protocol/protocol.dart' as protocol
     show Element, ElementKind;
@@ skipping 350 matching lines @@
 
   /**
    * Return a [Future] that completes with the containing library information
    * after it is accessible via [context.getLibrariesContaining].
    */
   Future computeLibrariesContaining([int times = 200]) {
     List<Source> libraries = context.getLibrariesContaining(testSource);
     if (libraries.isNotEmpty) {
       return new Future.value(libraries);
     }
+    if (times == 0) {
+      fail('failed to determine libraries containing $testSource');
+    }
     context.performAnalysisTask();
     // We use a delayed future to allow microtask events to finish. The
     // Future.value or Future() constructors use scheduleMicrotask themselves and
     // would therefore not wait for microtask callbacks that are scheduled after
     // invoking this method.
     return new Future.delayed(
         Duration.ZERO, () => computeLibrariesContaining(times - 1));
   }
 
   Future computeSuggestions([int times = 200]) async {
@@ skipping 78 matching lines @@
   }
 
   @override
   void setUp() {
     super.setUp();
     index = createLocalMemoryIndex();
     searchEngine = new SearchEngineImpl(index);
     contributor = createContributor();
   }
 }