Unify lots of copied code in one utility method

pkg/analyzer_plugin/lib/plugin/navigation_mixin.dart

Index: pkg/analyzer_plugin/lib/plugin/navigation_mixin.dart
diff --git a/pkg/analyzer_plugin/lib/plugin/navigation_mixin.dart b/pkg/analyzer_plugin/lib/plugin/navigation_mixin.dart
index 5bf7ccfad08c8963fecf754aaebc515a95e12fc1..9a2d3b41016c44eabb0092e3e37fc10e258b7893 100644
--- a/pkg/analyzer_plugin/lib/plugin/navigation_mixin.dart
+++ b/pkg/analyzer_plugin/lib/plugin/navigation_mixin.dart
@@ -28,13 +28,7 @@ abstract class DartNavigationMixin implements NavigationMixin {
   Future<NavigationRequest> getNavigationRequest(
       AnalysisGetNavigationParams 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 DartNavigationRequestImpl(
         resourceProvider, parameters.offset, parameters.length, result);
   }
@@ -57,6 +51,8 @@ abstract class NavigationMixin implements ServerPlugin {
   /**
    * Return the navigation request that should be passes to the contributors
    * returned from [getNavigationContributors].
+   *
+   * Throw a [RequestFailure] if the request could not be created.
    */
   Future<NavigationRequest> getNavigationRequest(
       AnalysisGetNavigationParams parameters);