[ddc] Do not require a @JS annotation at the library level.

Dart2js allows to declare JS-interp members, even if the enclosing library
doesn't have a JS annotation. This relaxes DDC to do the same.

Change-Id: I733d1cbb308692d89b8cd443cbde0ed30637e48e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/177780
Reviewed-by: Srujan Gaddam <srujzs@google.com>
Commit-Queue: Sigmund Cherem <sigmund@google.com>

pkg/dev_compiler/lib/src/kernel/js_interop.dart

@@ -120,14 +120,18 @@ bool hasJSInteropAnnotation(Class c) => c.annotations.any(isPublicJSAnnotation);
 
 /// Returns true iff this element is a JS interop member.
 ///
-/// The element's library must have `@JS(...)` annotation from `package:js`.
-/// If the element is a class, it must also be marked with `@JS`. Other
-/// elements, such as class members and top-level functions/accessors, should
-/// be marked `external`.
+/// JS annotations are required explicitly on classes. Other elements, such as
+/// class members and top-level functions/accessors, should be marked `external`
+/// and should have directly or indirectly a `JS` annotation. It is sufficient
+/// if the annotation is in the procedure itself or an enclosing element like
+/// the class or library.
 bool usesJSInterop(NamedNode n) {
-  var library = getLibrary(n);
-  return library != null &&
-      library.annotations.any(isPublicJSAnnotation) &&
-      (n is Procedure && n.isExternal ||
-          n is Class && n.annotations.any(isPublicJSAnnotation));
+  if (n is Member && n.isExternal) {
+    return n.enclosingLibrary.annotations.any(isPublicJSAnnotation) ||
+        n.annotations.any(isPublicJSAnnotation) ||
+        (n.enclosingClass?.annotations?.any(isPublicJSAnnotation) ?? false);
+  } else if (n is Class) {
+    return n.annotations.any(isPublicJSAnnotation);
+  }
+  return false;
 }

tests/lib/js/no_library_annotation_test.dart

@@ -0,0 +1,15 @@
+// Copyright (c) 2021, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Test that JS-interop works without a @JS annotation on the library itself.
+
+import 'package:js/js.dart';
+import 'package:expect/expect.dart';
+
+@JS()
+external dynamic eval(String code);
+
+void main() {
+  Expect.equals(2, eval("2"));
+}

tests/lib_2/js/no_library_annotation_test.dart

@@ -0,0 +1,17 @@
+// Copyright (c) 2021, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// @dart = 2.9
+
+// Test that JS-interop works without a @JS annotation on the library itself.
+
+import 'package:js/js.dart';
+import 'package:expect/expect.dart';
+
+@JS()
+external dynamic eval(String code);
+
+void main() {
+  Expect.equals(2, eval("2"));
+}