Elements. Deprecate PromotableElement.

Change-Id: I7484949ce7b3c0ce2bce9304224f639e8831bd02
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/418914
Reviewed-by: Paul Berry <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov

pkg/analyzer/api.txt

@@ -3468,7 +3468,7 @@ package:analyzer/dart/element/element.dart:
     imports (getter: List<LibraryImportElement>)
     name (getter: String)
     scope (getter: Scope)
-  PromotableElement (class extends Object implements LocalElement, VariableElement):
+  PromotableElement (class extends Object implements LocalElement, VariableElement, deprecated):
     new (constructor: PromotableElement Function())
     name (getter: String)
   PropertyAccessorElement (class extends Object implements ExecutableElement, deprecated):

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

@@ -2041,6 +2041,7 @@ abstract class PrefixElement implements _ExistingElement {
 /// variable or a parameter.
 ///
 /// Clients may not extend, implement or mix-in this class.
+@Deprecated('Use PromotableElement2 instead')
 abstract class PromotableElement implements LocalElement, VariableElement {
   // Promotable elements are guaranteed to have a name.
   @override
@@ -2103,10 +2104,8 @@ abstract class PropertyAccessorElement implements ExecutableElement {
 /// * Every explicit variable is represented by a non-synthetic
 ///   [PropertyInducingElement].
 /// * Every explicit variable induces a getter and possibly a setter, both of
-// ignore:deprecated_member_use_from_same_package
 ///   which are represented by synthetic [PropertyAccessorElement]s.
 /// * Every explicit getter or setter is represented by a non-synthetic
-// ignore:deprecated_member_use_from_same_package
 ///   [PropertyAccessorElement].
 /// * Every explicit getter or setter (or pair thereof if they have the same
 ///   name) induces a variable that is represented by a synthetic