Elements. Deprecate InstanceElement.

Change-Id: I7e1d80c01d90e64cc0e4b43a7ed58ceb7f149883
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/418941
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>

pkg/analyzer/api.txt

@@ -3329,7 +3329,7 @@ package:analyzer/dart/element/element.dart:
   ImportElementPrefix (class extends Object):
     new (constructor: ImportElementPrefix Function())
     element (getter: PrefixElement)
-  InstanceElement (class extends Object implements TypeDefiningElement, TypeParameterizedElement):
+  InstanceElement (class extends Object implements TypeDefiningElement, TypeParameterizedElement, deprecated):
     new (constructor: InstanceElement Function())
     accessors (getter: List<PropertyAccessorElement>)
     enclosingElement3 (getter: CompilationUnitElement, deprecated)

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

@@ -1320,6 +1320,7 @@ abstract class ImportElementPrefix {
 /// An element that has `this`.
 ///
 /// Clients may not extend, implement or mix-in this class.
+@Deprecated('Use InstanceElement2 instead')
 abstract class InstanceElement
     implements TypeDefiningElement, TypeParameterizedElement {
   /// The declared accessors (getters and setters).

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

@@ -5546,8 +5546,13 @@ class ImportElementPrefixImpl implements ImportElementPrefix {
 }
 
 abstract class InstanceElementImpl extends _ExistingElementImpl
-    with AugmentableFragment, TypeParameterizedElementMixin
-    implements InstanceElement, InstanceFragment {
+    with
+        AugmentableFragment,
+        TypeParameterizedElementMixin
+    implements
+        // ignore:deprecated_member_use_from_same_package
+        InstanceElement,
+        InstanceFragment {
   @override
   ElementLinkedData? linkedData;
 

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

@@ -459,18 +459,6 @@ extension GetterElementImplExtension on GetterElementImpl {
   }
 }
 
-extension InstanceElement2Extension on InstanceElement2 {
-  InstanceElement get asElement {
-    return firstFragment as InstanceElement;
-  }
-}
-
-extension InstanceElementExtension on InstanceElement {
-  InstanceElement2 get asElement2 {
-    return (this as InstanceElementImpl).element;
-  }
-}
-
 extension InstanceElementImpl2Extension on InstanceElementImpl2 {
   InstanceElementImpl get asElement {
     return firstFragment;