Support user generated custom native JS classes.

pkg/compiler/lib/src/types/types.dart

Index: pkg/compiler/lib/src/types/types.dart
diff --git a/pkg/compiler/lib/src/types/types.dart b/pkg/compiler/lib/src/types/types.dart
index a06404b12d2db3c3c5229b693da3f3346aa49c61..224c8b560247d6beedc13a0b14bcada2ab4ee10d 100644
--- a/pkg/compiler/lib/src/types/types.dart
+++ b/pkg/compiler/lib/src/types/types.dart
@@ -295,6 +295,9 @@ class TypesTask extends CompilerTask {
    */
   TypeMask getGuaranteedTypeOfElement(Element element) {
     return measure(() {
+      if (element.isJsInterop) {
+        return dynamicType;
+      }
       TypeMask guaranteedType = typesInferrer.getTypeOfElement(element);
       return (concreteTypesInferrer == null)
           ? guaranteedType
@@ -306,6 +309,10 @@ class TypesTask extends CompilerTask {
 
   TypeMask getGuaranteedReturnTypeOfElement(Element element) {
     return measure(() {
+      if (element.isJsInterop) {
+        return dynamicType;
+      }
+
       TypeMask guaranteedType =
           typesInferrer.getReturnTypeOfElement(element);
       return (concreteTypesInferrer == null)