Unify lots of copied code in one utility method

pkg/analyzer_plugin/lib/plugin/kythe_mixin.dart

Index: pkg/analyzer_plugin/lib/plugin/kythe_mixin.dart
diff --git a/pkg/analyzer_plugin/lib/plugin/kythe_mixin.dart b/pkg/analyzer_plugin/lib/plugin/kythe_mixin.dart
index b790cfa84851695320e600bf0e4f516fa544d41f..3db9fbb31dd8951bdd7295e30d3c99895e307912 100644
--- a/pkg/analyzer_plugin/lib/plugin/kythe_mixin.dart
+++ b/pkg/analyzer_plugin/lib/plugin/kythe_mixin.dart
@@ -28,13 +28,7 @@ abstract class DartEntryMixin implements EntryMixin {
   Future<EntryRequest> getEntryRequest(
       KytheGetKytheEntriesParams 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 DartEntryRequestImpl(resourceProvider, result);
   }
 }
@@ -56,6 +50,8 @@ abstract class EntryMixin implements ServerPlugin {
   /**
    * Return the entries request that should be passes to the contributors
    * returned from [getEntryContributors].
+   *
+   * Throw a [RequestFailure] if the request could not be created.
    */
   Future<EntryRequest> getEntryRequest(KytheGetKytheEntriesParams parameters);