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 6b1898117696ba0fbfc9516b144b25655dcb60ca..94113d7ca5c7ce5e29d1fc46506e9c026849a211 100644
--- a/pkg/compiler/lib/src/types/types.dart
+++ b/pkg/compiler/lib/src/types/types.dart
@@ -289,6 +289,10 @@ class TypesTask extends CompilerTask {
    */
   TypeMask getGuaranteedTypeOfElement(Element element) {
     return measure(() {
+      // TODO(24489): trust some JsInterop types.
+      if (element.isJsInterop) {
+        return dynamicType;
+      }
       TypeMask guaranteedType = typesInferrer.getTypeOfElement(element);
       return guaranteedType;
     });
@@ -296,6 +300,11 @@ class TypesTask extends CompilerTask {
 
   TypeMask getGuaranteedReturnTypeOfElement(Element element) {
     return measure(() {
+      // TODO(24489): trust some JsInterop types.
+      if (element.isJsInterop) {
+        return dynamicType;
+      }
+
       TypeMask guaranteedType =
           typesInferrer.getReturnTypeOfElement(element);
       return guaranteedType;