[element model] migrate await_only_futures

Bug: https://github.com/dart-lang/linter/issues/5099
Change-Id: I280d5b1a93a0944208c976a51e6d0be007e2e4aa
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/387325
Commit-Queue: Brian Wilkerson <[email protected]>
Auto-Submit: Phil Quitslund <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: pq

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

@@ -43,7 +43,10 @@ class DynamicTypeImpl extends TypeImpl
   DynamicTypeImpl._();
 
   @override
-  Element2? get element3 => (element as Fragment).element;
+  Element2? get element3 => switch (element) {
+        Fragment(:var element) => element,
+        _ => null,
+      };
 
   @override
   int get hashCode => 1;

pkg/linter/lib/src/rules/await_only_futures.dart

@@ -4,7 +4,7 @@
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
-import 'package:analyzer/dart/element/element.dart';
+import 'package:analyzer/dart/element/element2.dart';
 import 'package:analyzer/dart/element/type.dart';
 
 import '../analyzer.dart';
@@ -41,7 +41,7 @@ class _Visitor extends SimpleAstVisitor<void> {
 
     var type = node.expression.staticType;
     if (!(type == null ||
-        type.element is ExtensionTypeElement ||
+        type.element3 is ExtensionTypeElement2 ||
         type.isDartAsyncFuture ||
         type is DynamicType ||
         type is InvalidType ||