Add FindElement2 as a replacement for FindElement in tests

If it would be helpful I can redo this CL with two patchsets, one that
adds the new class as a copy of the old and a second that makes all of
the changes to it.

Change-Id: Ie7a044fbd08d12fde1c942e54f6927e0cf95cacd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/390921
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>

Author: bwilkerson

pkg/analyzer/lib/dart/element/element2.dart

@@ -51,6 +51,7 @@ import 'package:analyzer/dart/constant/value.dart';
 import 'package:analyzer/dart/element/element.dart'
     show
         DirectiveUri,
+        DirectiveUriWithUnit,
         ElementAnnotation,
         ElementKind,
         ElementLocation,
@@ -2304,3 +2305,8 @@ abstract class VariableFragment implements Fragment {
   /// even though they are implicitly final.
   bool get isFinal;
 }
+
+extension DirectiveUriWithUnitExtension on DirectiveUriWithUnit {
+  /// The library fragment associated with this directive.
+  LibraryFragment get libraryFragment => unit as LibraryFragment;
+}