Support user generated custom native JS classes.

pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart

Index: pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
diff --git a/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart b/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
index fccfb8042bee4f2fb745432b3d309880a72bc3d1..ac375a2bc2eaf23632e1bc8a07c59188ebbe63fe 100644
--- a/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
+++ b/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
@@ -452,9 +452,10 @@ abstract class InferrerEngine<T, V extends TypeSystem>
   }
 
   bool isNativeElement(Element element) {
-    if (element.isNative) return true;
+    if (element.isNative || element.isJsInterop) return true;
     return element.isClassMember
-        && element.enclosingClass.isNative
+        && (element.enclosingClass.isNative
+            || element.enclosingClass.isJsInterop)
         && element.isField;
   }
 
@@ -556,7 +557,7 @@ class SimpleTypeInferrerVisitor<T>
       inferrer.setDefaultTypeOfParameter(element, type);
     });
 
-    if (analyzedElement.isNative) {
+    if (analyzedElement.isNative || analyzedElement.isJsInterop) {
       // Native methods do not have a body, and we currently just say
       // they return dynamic.
       return types.dynamicType;