Bug 920840 - Crash when JS-implemented WebIDL's init method returns a value other than undefined. r=bz

dom/bindings/BindingUtils.cpp

@@ -2040,8 +2040,13 @@ ConstructJSImplementation(JSContext* aCx, const char* aContractId,
         aRv.Throw(rv);
         return nullptr;
       }
-      MOZ_ASSERT(initReturn.isUndefined(),
-                 "nsIDOMGlobalPropertyInitializer should return undefined");
+      // With JS-implemented WebIDL, the return value of init() is not used to determine
+      // if init() failed, so init() should only return undefined. Any kind of permission
+      // or pref checking must happen by adding an attribute to the WebIDL interface.
+      if (!initReturn.isUndefined()) {
+        MOZ_ASSERT(false, "The init() method for JS-implemented WebIDL should not return anything");
+        MOZ_CRASH();
+      }
     }
     // Extract the JS implementation from the XPCOM object.
     nsCOMPtr<nsIXPConnectWrappedJS> implWrapped =