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..8f0f26b169853be4fbcac6fef2ff4efa52a5a480 100644
--- a/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
+++ b/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
@@ -454,7 +454,8 @@ abstract class InferrerEngine<T, V extends TypeSystem>
   bool isNativeElement(Element element) {
     if (element.isNative) return true;
     return element.isClassMember
-        && element.enclosingClass.isNative
+        && (element.enclosingClass.isNative
+            || element.enclosingClass.isJsInterop)
         && element.isField;
   }
 
@@ -1610,6 +1611,9 @@ class SimpleTypeInferrerVisitor<T>
     // we have to forward the call to the effective target of the
     // factory.
     if (element.isFactoryConstructor) {
+
+      if (element.isJsInterop) return types.dynamicType;
+
       // TODO(herhut): Remove the while loop once effectiveTarget forwards to
       //               patches.
       while (element.isFactoryConstructor) {
@@ -1621,6 +1625,7 @@ class SimpleTypeInferrerVisitor<T>
     if (compiler.backend.isForeign(element)) {
       return handleForeignSend(node, element);
     }
+
     Selector selector = elements.getSelector(node);
     TypeMask mask = elements.getTypeMask(node);
     // In erroneous code the number of arguments in the selector might not
@@ -2094,7 +2099,9 @@ class SimpleTypeInferrerVisitor<T>
     assert(!element.isFactoryConstructor ||
            !(element as ConstructorElement).isRedirectingFactory);
     // Erroneous elements may be unresolved, for example missing getters.
+
     if (Elements.isUnresolved(element)) return types.dynamicType;
+
     // TODO(herhut): should we follow redirecting constructors here? We would
     // need to pay attention if the constructor is pointing to an erroneous
     // element.
@@ -2124,6 +2131,7 @@ class SimpleTypeInferrerVisitor<T>
       ast.Node receiver = send.receiver;
       if (receiver != null) {
         Element element = elements[receiver];
+
         if (Elements.isLocal(element) && !capturedVariables.contains(element)) {
           T refinedType = types.refineReceiver(
               selector, mask, receiverType, send.isConditional);