kludges: Add overrideWithFallback

kludges.js

@@ -244,6 +244,20 @@ function registerOverridePrototype(obj, name, override) {
     registerOverrideProp(obj.prototype, name, override);
 }
 
+function makeFallback(obj, method, ...args) {
+    let fallback = getSavedPrototype(obj, method);
+    return fallback.bind(...args);
+}
+
+function overrideWithFallback(obj, method, body) {
+    registerOverridePrototype(
+        obj, method, function(...args) {
+            let fallback = makeFallback(obj, method, this, ...args);
+            body(fallback, this, ...args);
+        }
+    );
+}
+
 function getSavedProp(obj, name) {
     let props = savedProps.get(obj);
     if (!props)