Elements. Deprecate MixinElement.

Change-Id: I03ded956186834561e2ab1ce6ff0626c9f054e2e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/418827
Reviewed-by: Paul Berry <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov

pkg/analyzer/api.txt

@@ -3423,7 +3423,7 @@ package:analyzer/dart/element/element.dart:
   MethodElement (class extends Object implements ClassMemberElement, ExecutableElement):
     new (constructor: MethodElement Function())
     declaration (getter: MethodElement)
-  MixinElement (class extends Object implements InterfaceElement):
+  MixinElement (class extends Object implements InterfaceElement, deprecated):
     new (constructor: MixinElement Function())
     isBase (getter: bool)
     superclassConstraints (getter: List<InterfaceType>)

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

@@ -1832,6 +1832,7 @@ abstract class MethodElement
 /// An element that represents a mixin.
 ///
 /// Clients may not extend, implement or mix-in this class.
+@Deprecated('Use MixinElement2 instead')
 abstract class MixinElement implements InterfaceElement {
   /// Whether the mixin is a base mixin.
   ///

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

@@ -1199,6 +1199,7 @@ class CompilationUnitElementImpl extends UriReferencedElementImpl
   /// Returns the mixin defined in this compilation unit that has the given
   /// [name], or `null` if this compilation unit does not define a mixin with
   /// the given name.
+  @Deprecated(elementModelDeprecationMsg)
   MixinElement? getMixin(String name) {
     for (var mixin in mixins) {
       if (mixin.name == name) {
@@ -7370,6 +7371,7 @@ class LibraryElementImpl extends ElementImpl
     return _getElementByName(getters, name);
   }
 
+  @Deprecated(elementModelDeprecationMsg)
   MixinElement? getMixin(String name) {
     for (var unitElement in units) {
       var element = unitElement.getMixin(name);
@@ -8436,7 +8438,10 @@ abstract class MethodElementOrMember
 
 /// A [ClassElementImpl] representing a mixin declaration.
 class MixinElementImpl extends ClassOrMixinElementImpl
-    implements MixinElement, MixinFragment {
+    implements
+        // ignore:deprecated_member_use_from_same_package
+        MixinElement,
+        MixinFragment {
   List<InterfaceTypeImpl> _superclassConstraints = const [];
 
   /// Names of methods, getters, setters, and operators that this mixin

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

@@ -656,18 +656,6 @@ extension MethodElementOrMemberExtension on MethodElementOrMember {
   }
 }
 
-extension MixinElement2Extension on MixinElement2 {
-  MixinElement get asElement {
-    return firstFragment as MixinElement;
-  }
-}
-
-extension MixinElementExtension on MixinElement {
-  MixinElement2 get asElement2 {
-    return (this as MixinElementImpl).element;
-  }
-}
-
 extension MixinElementImplExtension on MixinElementImpl {
   MixinElementImpl2 get asElement2 {
     return element;