Elements. Deprecate Element.

Change-Id: Ibc46feb65931e12677330ec012507158747f6646
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/419820
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer/api.txt

@@ -3095,7 +3095,7 @@ package:analyzer/dart/element/element.dart:
     new (constructor: DirectiveUriWithUnit Function())
     libraryFragment (getter: LibraryFragment, experimental)
     unit (getter: CompilationUnitElement, deprecated)
-  Element (class extends Object implements AnalysisTarget):
+  Element (class extends Object implements AnalysisTarget, deprecated):
     new (constructor: Element Function())
     children (getter: List<Element>, deprecated)
     context (getter: AnalysisContext@2)

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

@@ -408,7 +408,7 @@ abstract class DirectiveUriWithUnit extends DirectiveUriWithSource {
 /// represent the semantic structure of the program.
 ///
 /// Clients may not extend, implement or mix-in this class.
-// ignore: deprecated_member_use_from_same_package
+@Deprecated(elementModelDeprecationMsg)
 abstract class Element implements AnalysisTarget {
   /// A list of this element's children.
   ///

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

@@ -2545,7 +2545,7 @@ class ElementAnnotationImpl implements ElementAnnotation {
 
 abstract class ElementImpl
     implements
-        // ignore:analyzer_use_new_elements
+        // ignore:deprecated_member_use_from_same_package,analyzer_use_new_elements
         Element,
         ElementOrMember {
   static const _metadataFlag_isReady = 1 << 0;

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

@@ -787,7 +787,7 @@ class GetterMember extends PropertyAccessorMember
 /// parameters are known.
 abstract class Member
     implements
-        // ignore:analyzer_use_new_elements
+        // ignore:deprecated_member_use_from_same_package,analyzer_use_new_elements
         Element,
         ElementOrMember {
   /// The element on which the parameterized element was created.