[element model] migrate unnecessary_await_in_return

Bug: https://github.com/dart-lang/linter/issues/5099
Change-Id: I0e203cd6fbc3006f84ccd99e8244a3e185c9f0ca
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/391205
Commit-Queue: Brian Wilkerson <[email protected]>
Auto-Submit: Phil Quitslund <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: pq

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

@@ -64,9 +64,9 @@ class _Visitor extends SimpleAstVisitor<void> {
 
     DartType? returnType;
     if (parent is FunctionExpression) {
-      returnType = parent.declaredElement?.returnType;
+      returnType = parent.declaredFragment?.element.returnType;
     } else if (parent is MethodDeclaration) {
-      returnType = parent.declaredElement?.returnType;
+      returnType = parent.declaredFragment?.element.returnType;
     } else if (parent is Block) {
       // removing await in try block changes the behaviour
       return;