Wire up class hierarchy for recursive inheritance

tool/input_sdk/private/ddc_runtime/classes.dart

Index: tool/input_sdk/private/ddc_runtime/classes.dart
diff --git a/tool/input_sdk/private/ddc_runtime/classes.dart b/tool/input_sdk/private/ddc_runtime/classes.dart
index 9359acea9e367fcaf83aaec2caa6efa36612ef1e..d8b06e6cd72ca795d814f50db976806ffe237678 100644
--- a/tool/input_sdk/private/ddc_runtime/classes.dart
+++ b/tool/input_sdk/private/ddc_runtime/classes.dart
@@ -427,11 +427,14 @@ list(obj, elementType) =>
 /// Link the extension to the type it's extending as a base class.
 setBaseClass(derived, base) {
   JS('', '#.prototype.__proto__ = #.prototype', derived, base);
+  // We use __proto__ to track the superclass hierarchy (see isSubtype).
+  JS('', '#.__proto__ = #', derived, base);
 }
 
 /// Like [setBaseClass] but for generic extension types, e.g. `JSArray<E>`
 setExtensionBaseClass(derived, base) {
   // Mark the generic type as an extension type.
   JS('', '#.prototype[#] = #', derived, _extensionType, derived);
-  setBaseClass(derived, base);
+  // Link the prototype objects
+  JS('', '#.prototype.__proto__ = #.prototype', derived, base);
 }