Elements. Make ElementDirective abstract, not sealed.

This allows to have `ElementDirectiveImpl` internally.

Presubmit in google3 is green: cl/752778004

Change-Id: Ia56ad45fab8d95aa5a089aae338414a7f5c4d549
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/425340
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer/CHANGELOG.md

@@ -9,6 +9,9 @@
 * Deprecate `AnalysisError.correction` field; use
   `AnalysisError.correctionMessage` instead.
 * Deprecate `ErrorType`; use `DiagnosticType` instead.
+* Change `ElementDirective` from `sealed` to `abstract`.
+  This allows the analyzer to have an internal implementation
+  class corresponding to `ElementDirective`.
 
 ## 7.4.1
 * Restore `InstanceElement.augmented` getter.

pkg/analyzer/api.txt

@@ -3115,7 +3115,8 @@ package:analyzer/dart/element/element.dart:
     isWidgetFactory (getter: bool)
     computeConstantValue (method: DartObject? Function())
     toSource (method: String Function())
-  ElementDirective (class extends Object implements Annotatable, sealed (immediate subtypes: LibraryExport, LibraryImport, PartInclude)):
+  ElementDirective (class extends Object implements Annotatable):
+    new (constructor: ElementDirective Function())
     libraryFragment (getter: LibraryFragment)
     uri (getter: DirectiveUri)
   ElementKind (class extends Object implements Comparable<ElementKind>):

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

@@ -845,7 +845,7 @@ abstract class ElementAnnotation implements ConstantEvaluationTarget {
 /// A directive within a library fragment.
 ///
 /// Clients may not extend, implement or mix-in this class.
-sealed class ElementDirective implements Annotatable {
+abstract class ElementDirective implements Annotatable {
   /// The library fragment that contains this object.
   LibraryFragment get libraryFragment;