Support user generated custom native JS classes.

pkg/compiler/lib/src/elements/elements.dart

Index: pkg/compiler/lib/src/elements/elements.dart
diff --git a/pkg/compiler/lib/src/elements/elements.dart b/pkg/compiler/lib/src/elements/elements.dart
index 219349be5c646947254023dc13e9b68beea372ef..408c27408e5ee4f25b25fcb16e4433ea8c242c87 100644
--- a/pkg/compiler/lib/src/elements/elements.dart
+++ b/pkg/compiler/lib/src/elements/elements.dart
@@ -300,6 +300,8 @@ abstract class Element implements Entity {
   bool get isTopLevel;
   bool get isAssignable;
   bool get isNative;
+  bool get isJsInterop;
+
   bool get isDeferredLoaderGetter;
 
   /// True if the element is declared in a patch library but has no
@@ -396,6 +398,8 @@ abstract class Element implements Entity {
   bool get hasFixedBackendName;
   String get fixedBackendName;
 
+  String get fixedBackendReceiver;
+
   bool get isAbstract;
 
   Scope buildScope();
@@ -407,6 +411,8 @@ abstract class Element implements Entity {
   AnalyzableElement get analyzableElement;
 
   accept(ElementVisitor visitor, arg);
+
+  void setJsInterop(String name);
 }
 
 class Elements {
@@ -511,7 +517,7 @@ class Elements {
 
   static bool isNativeOrExtendsNative(ClassElement element) {
     if (element == null) return false;
-    if (element.isNative) return true;
+    if (element.isNative || element.isJsInterop) return true;
     assert(element.isResolved);
     return isNativeOrExtendsNative(element.superclass);
   }