Add @anonymous annotation and restrict object literal constructors to only anonymous classes. This …

pkg/js/lib/js.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.
 
 /// Allows interoperability with Javascript APIs.
 library js;
 
 export 'dart:js' show allowInterop, allowInteropCaptureThis;
 
 /// A metadata annotation that indicates that a Library, Class, or member is
 /// implemented directly in JavaScript. All external members of a class or
 /// library with this annotation implicitly have it as well.
 ///
 /// Specifying [name] customizes the JavaScript name to use. By default the
 /// dart name is used. It is not valid to specify a custom [name] for class
 /// instance members.
 class JS {
   final String name;
   const JS([this.name]);
 }
+
+class _Anonymous {
+  const _Anonymous();
+}
+
+/// A metadata annotation that indicates that a @JS annotated class is
+/// structural and does not have a known JavaScript prototype.
+///
+/// Factory constructors for anonymous JavaScript classes desugar to creating
+/// JavaScript object literals with name-value pairs corresponding to the
+/// parameter names and values.
+const _Anonymous anonymous = const _Anonymous();