Unify lots of copied code in one utility method

pkg/analyzer_plugin/lib/plugin/fix_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/error/error.dart';
 import 'package:analyzer/src/dart/analysis/driver.dart';
 import 'package:analyzer/src/generated/source.dart';
@@ skipping 11 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
  * [FixesMixin] as a mix-in.
  */
 abstract class DartFixesMixin implements FixesMixin {
   @override
   Future<FixesRequest> getFixesRequest(EditGetFixesParams parameters) async {
     String path = parameters.file;
     int offset = parameters.offset;
-    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 DartFixesRequestImpl(
         resourceProvider, offset, _getErrors(offset, result), result);
   }
 
   List<AnalysisError> _getErrors(int offset, ResolveResult result) {
     LineInfo lineInfo = result.lineInfo;
     int offsetLine = lineInfo.getLocation(offset).lineNumber;
     return result.errors.where((AnalysisError error) {
       int errorLine = lineInfo.getLocation(error.offset).lineNumber;
       return errorLine == offsetLine;
@@ skipping 11 matching lines @@
 abstract class FixesMixin implements ServerPlugin {
   /**
    * Return a list containing the fix contributors that should be used to create
    * fixes for the file with the given [path].
    */
   List<FixContributor> getFixContributors(String path);
 
   /**
    * Return the fixes request that should be passes to the contributors
    * returned from [getFixContributors].
+   *
+   * Throw a [RequestFailure] if the request could not be created.
    */
   Future<FixesRequest> getFixesRequest(EditGetFixesParams parameters);
 
   @override
   Future<EditGetFixesResult> handleEditGetFixes(
       EditGetFixesParams parameters) async {
     String path = parameters.file;
     FixesRequest request = await getFixesRequest(parameters);
     FixGenerator generator = new FixGenerator(getFixContributors(path));
     GeneratorResult result = await generator.generateFixesResponse(request);
     result.sendNotifications(channel);
     return result.result;
   }
 }