Unify lots of copied code in one utility method

pkg/analyzer_plugin/lib/plugin/completion_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 CompletionMixin implements ServerPlugin {
   /**
    * Return a list containing the completion contributors that should be used to
    * create completion suggestions for the file with the given [path].
    */
   List<CompletionContributor> getCompletionContributors(String path);
 
   /**
    * Return the completion request that should be passes to the contributors
    * returned from [getCompletionContributors].
+   *
+   * Throw a [RequestFailure] if the request could not be created.
    */
   Future<CompletionRequest> getCompletionRequest(
       CompletionGetSuggestionsParams parameters);
 
   @override
   Future<CompletionGetSuggestionsResult> handleCompletionGetSuggestions(
       CompletionGetSuggestionsParams parameters) async {
     String path = parameters.file;
     CompletionRequest request = await getCompletionRequest(parameters);
     CompletionGenerator generator =
@@ skipping 13 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
  * [CompletionMixin] as a mix-in.
  */
 abstract class DartCompletionMixin implements CompletionMixin {
   @override
   Future<CompletionRequest> getCompletionRequest(
       CompletionGetSuggestionsParams 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(parameters.file);
+    ResolveResult result = await getResolveResult(parameters.file);
     return new DartCompletionRequestImpl(
         resourceProvider, parameters.offset, result);
   }
 }