Unify lots of copied code in one utility method

pkg/analyzer_plugin/lib/plugin/assist_mixin.dart

 // Copyright (c) 2017, 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.
 
 import 'dart:async';
 
 import 'package:analyzer/dart/analysis/results.dart';
 import 'package:analyzer/src/dart/analysis/driver.dart';
 import 'package:analyzer_plugin/plugin/plugin.dart';
 import 'package:analyzer_plugin/protocol/protocol.dart';
@@ skipping 12 matching lines @@
 abstract class AssistsMixin implements ServerPlugin {
   /**
    * Return a list containing the assist contributors that should be used to
    * create assists for the file with the given [path].
    */
   List<AssistContributor> getAssistContributors(String path);
 
   /**
    * Return the assist request that should be passes to the contributors
    * returned from [getAssistContributors].
+   *
+   * Throw a [RequestFailure] if the request could not be created.
    */
   Future<AssistRequest> getAssistRequest(EditGetAssistsParams parameters);
 
   @override
   Future<EditGetAssistsResult> handleEditGetAssists(
       EditGetAssistsParams parameters) async {
     String path = parameters.file;
     AssistRequest request = await getAssistRequest(parameters);
     AssistGenerator generator =
         new AssistGenerator(getAssistContributors(path));
@@ skipping 10 matching lines @@
  *
  * Clients may not extend or implement this class, but are allowed to use it as
  * a mix-in when creating a subclass of [ServerPlugin] that also uses
  * [AssistsMixin] as a mix-in.
  */
 abstract class DartAssistsMixin implements AssistsMixin {
   @override
   Future<AssistRequest> getAssistRequest(
       EditGetAssistsParams parameters) async {
     String path = parameters.file;
-    AnalysisDriver driver = driverForPath(path);
-    if (driver == null) {
-      // Return an error from the request.
-      throw new RequestFailure(
-          RequestErrorFactory.pluginError('Failed to analyze $path', null));
-    }
-    ResolveResult result = await driver.getResult(path);
+    ResolveResult result = await getResolveResult(path);
     return new DartAssistRequestImpl(
         resourceProvider, parameters.offset, parameters.length, result);
   }
 }