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..6fbd2b1c445b0cea18d21d47cf1aef5afdd7caff 100644
--- a/pkg/compiler/lib/src/elements/elements.dart
+++ b/pkg/compiler/lib/src/elements/elements.dart
@@ -300,6 +300,7 @@ 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
@@ -511,7 +512,7 @@ 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);
   }
@@ -1359,6 +1360,7 @@ abstract class ClassElement extends TypeDeclarationElement
   ClassElement get implementation;
 
   String get nativeTagInfo;
+  bool get isJsInterop;
 
   /// `true` if this class is an enum declaration.
   bool get isEnumClass;