implement @JS classes

lib/src/compiler/js_interop.dart

 // Copyright (c) 2015, 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.
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/src/generated/constant.dart';
 import 'package:analyzer/dart/element/element.dart';
 
 import 'element_helpers.dart';
 
@@ skipping 22 matching lines @@
 bool isJsSpreadInvocation(MethodInvocation i) =>
     _isJsLibType('spread', i.methodName?.bestElement);
 
 // TODO(jmesserly): Move JsPeerInterface to package:js (see issue #135).
 // TODO(jacobr): The 'JS' annotation is the new, publically accessible one.
 // The 'JsName' annotation is the old one using internally by dart2js and
 // html libraries.  These two concepts will probably merge eventually.
 bool isJSAnnotation(DartObjectImpl value) =>
     _isJsLibType('JS', value.type.element) || isJsName(value);
 
+/// Returns [true] if [e] is the `JS` annotation from `package:js`.
+bool isPublicJSAnnotation(DartObjectImpl value) =>
+    _isJsLibType('JS', value.type.element);
+
+bool isJSAnonymousAnnotation(DartObjectImpl value) =>
+    _isJsLibType('_Anonymous', value.type.element);
+
 bool _isBuiltinAnnotation(
     DartObjectImpl value, String libraryName, String annotationName) {
   var e = value?.type?.element;
   if (e?.name != annotationName) return false;
   var uri = e.source.uri;
   var path = uri.pathSegments[0];
   return uri.scheme == 'dart' && path == libraryName;
 }
 
 /// Whether [value] is a `@JSExportName` (internal annotation used in SDK
@@ skipping 12 matching lines @@
 
 /// Returns the name value of the `JSExportName` annotation (when compiling
 /// the SDK), or `null` if there's none. This is used to control the name
 /// under which functions are compiled and exported.
 String getJSExportName(Element e) {
   if (e.source.isInSystemLibrary) {
     return getAnnotationName(e, isJSExportNameAnnotation) ?? e.name;
   }
   return e.name;
 }