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

pkg/js/lib/js.dart

Index: pkg/js/lib/js.dart
diff --git a/pkg/js/lib/js.dart b/pkg/js/lib/js.dart
index 4cb53c5479752c2fa9740ef4eaae6b78fef0a870..17b1ded20fe6c3cf7890b21f642afb89befe0b89 100644
--- a/pkg/js/lib/js.dart
+++ b/pkg/js/lib/js.dart
@@ -18,3 +18,15 @@ 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();