Unify lots of copied code in one utility method

pkg/analyzer_plugin/lib/plugin/assist_mixin.dart

Index: pkg/analyzer_plugin/lib/plugin/assist_mixin.dart
diff --git a/pkg/analyzer_plugin/lib/plugin/assist_mixin.dart b/pkg/analyzer_plugin/lib/plugin/assist_mixin.dart
index ee0431b77750b1f0d3674a3c8d677b7fad3bad10..1b1d3a0cd49c93a17f186e0e7afcae009520b709 100644
--- a/pkg/analyzer_plugin/lib/plugin/assist_mixin.dart
+++ b/pkg/analyzer_plugin/lib/plugin/assist_mixin.dart
@@ -30,6 +30,8 @@ abstract class AssistsMixin implements ServerPlugin {
   /**
    * 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);
 
@@ -60,13 +62,7 @@ abstract class DartAssistsMixin implements AssistsMixin {
   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);
   }