Elements. Deprecate PartElement.

Change-Id: I623ddbd3335e06222095cf7eaeae0409f2f423d0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/418918
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer/api.txt

@@ -3456,7 +3456,7 @@ package:analyzer/dart/element/element.dart:
     parameters (getter: List<ParameterElement>)
     typeParameters (getter: List<TypeParameterElement>, deprecated)
     appendToWithoutDelimiters (method: void Function(StringBuffer, {bool withNullability}))
-  PartElement (class extends Object implements _ExistingElement):
+  PartElement (class extends Object implements _ExistingElement, deprecated):
     new (constructor: PartElement Function())
     uri (getter: DirectiveUri)
   PatternVariableElement (class extends Object implements LocalVariableElement, deprecated):

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

@@ -2007,6 +2007,7 @@ abstract class ParameterElement
 /// A 'part' directive within a library.
 ///
 /// Clients may not extend, implement or mix-in this class.
+@Deprecated('Use PartInclude instead')
 abstract class PartElement implements _ExistingElement {
   /// The interpretation of the URI specified in the directive.
   DirectiveUri get uri;

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

@@ -9655,7 +9655,10 @@ mixin ParameterElementMixin
 }
 
 class PartElementImpl extends _ExistingElementImpl
-    implements PartElement, PartInclude {
+    implements
+        // ignore:deprecated_member_use_from_same_package
+        PartElement,
+        PartInclude {
   @override
   final DirectiveUriImpl uri;