Type-related fixes

pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/operations.dart

Index: pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/operations.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/operations.dart b/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/operations.dart
index 946507ca217b5c90b615dd971660901c0b6d3d60..5553144bf0effbe623e99fb54bb84ac1b948a770 100644
--- a/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/operations.dart
+++ b/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/operations.dart
@@ -795,7 +795,9 @@ runtimeType(obj) {
   // Delegate to the (possibly user-defined) method on the object.
   var extension = getExtensionType(obj);
   if (extension != null) {
-    return JS('', '#[dartx.runtimeType]', obj);
+    result = JS('', '#[dartx.runtimeType]', obj);
+    // If extension doesn't override runtimeType, return the extension type.
+    return result ?? wrapType(extension);
   }
   if (JS('bool', 'typeof # == "function"', obj)) {
     return wrapType(getReifiedType(obj));