extension type support for await_only_futures

Fixes: https://github.com/dart-lang/linter/issues/4685

Change-Id: I7cbc1a2a99384d7bc049e4e98ce3de53209398dc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/322566
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Phil Quitslund <[email protected]>

Author: pq

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

@@ -4,6 +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/type.dart';
 
 import '../analyzer.dart';
@@ -70,6 +71,7 @@ class _Visitor extends SimpleAstVisitor<void> {
 
     var type = node.expression.staticType;
     if (!(type == null ||
+        type.element is ExtensionTypeElement ||
         type.isDartAsyncFuture ||
         type is DynamicType ||
         type is InvalidType ||

pkg/linter/test/rules/await_only_futures_test.dart

@@ -17,6 +17,29 @@ class AwaitOnlyFuturesTest extends LintRuleTest {
   @override
   String get lintRule => 'await_only_futures';
 
+  test_extensionType_implementingFuture() async {
+    await assertNoDiagnostics(r'''
+extension type E(Future f) implements Future { }
+
+void f() async {
+  await E(Future.value());
+}
+''');
+  }
+
+  test_extensionType_notImplementingFuture() async {
+    await assertDiagnostics(r'''
+extension type E(int c) { }
+
+void f() async {
+  await E(1);
+}
+''', [
+      // No lint
+      error(CompileTimeErrorCode.AWAIT_OF_EXTENSION_TYPE_NOT_FUTURE, 48, 5),
+    ]);
+  }
+
   test_undefinedClass() async {
     await assertDiagnostics(r'''
 Undefined f() async => await f();