[analyzer] Fix PartElementImpl.enclosingUnit.

`PartElementImpl.enclosingElement3` has static type `Element?`, but in
practice it is always of type `CompilationUnitElementImpl`, due to
this code from `CompilationUnitElementImpl.parts=`:

    set parts(List<PartElementImpl> parts) {
      for (var part in parts) {
        part.enclosingElement3 = this;
        var uri = part.uri;
        if (uri is DirectiveUriWithUnitImpl) {
          uri.unit.enclosingElement3 = this;
        }
      }
      _parts = parts;
    }

Therefore, `PartElementImpl.enclosingUnit` can just cast
`enclosingElement3` to the proper type and return it.

This behavior was untested, so I've added a test in
`part_test.dart`. Without the fix, the test throws a `TypeError`.

Change-Id: I3e6f186826aa92cda9c3887c85c5eddc1fea7f31
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/408641
Reviewed-by: Konstantin Shcheglov <[email protected]>
Auto-Submit: Paul Berry <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: stereotype441

pkg/analyzer/lib/src/dart/element/element.dart

@@ -9653,10 +9653,8 @@ class PartElementImpl extends _ExistingElementImpl
   }
 
   @override
-  CompilationUnitElementImpl get enclosingUnit {
-    var enclosingLibrary = enclosingElement3 as LibraryElementImpl;
-    return enclosingLibrary._definingCompilationUnit;
-  }
+  CompilationUnitElementImpl get enclosingUnit =>
+      enclosingElement3 as CompilationUnitElementImpl;
 
   @override
   String get identifier => 'part';

pkg/analyzer/test/src/dart/resolution/part_test.dart

@@ -2,8 +2,10 @@
 // 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 'package:analyzer/src/dart/element/element.dart';
 import 'package:analyzer/src/dart/error/syntactic_errors.dart';
 import 'package:analyzer/src/error/codes.dart';
+import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 import 'context_collection_resolution.dart';
@@ -16,6 +18,19 @@ main() {
 
 @reflectiveTest
 class PartDirectiveResolutionTest extends PubPackageResolutionTest {
+  test_enclosingUnit() async {
+    newFile('$testPackageLibPath/a.dart', r'''
+part of 'test.dart';
+''');
+
+    await assertNoErrorsInCode(r'''
+part 'a.dart';
+''');
+    expect(
+        (findNode.part('part').partInclude! as PartElementImpl).enclosingUnit,
+        same(findNode.unit.declaredFragment));
+  }
+
   test_inLibrary_configurations_default() async {
     newFile('$testPackageLibPath/a.dart', r'''
 part of 'test.dart';