feat(csr): diagnose missing output binding functions

packages/csr/src/reactive-runtime.ts

@@ -210,6 +210,17 @@ export const REACTIVE_RUNTIME = String.raw`
       // CustomEvent can be unavailable in minimal DOM test environments.
     }
   }
+
+  function warnMissingBindingFunction(statement, resolve, element) {
+    var match = /^\s*([A-Za-z_$][\w$]*)\s*\(/.exec(statement || "");
+    if (!match || typeof resolve(match[1]) === "function") return;
+    warnOnce(
+      "WRN-DEV-BINDING-MISSING",
+      "Component binding calls '" + match[1] + "', but that function does not exist in the parent scope.",
+      element,
+      { binding: statement, functionName: match[1] },
+    );
+  }
   /*__WRNEXUS_DEV_END__*/
 
   function scheduleUpdateHook(element, callback) {
@@ -2514,6 +2525,9 @@ export const REACTIVE_RUNTIME = String.raw`
             locals.event = undefined;
             locals.$event = undefined;
             try {
+              /*__WRNEXUS_DEV_START__*/
+              warnMissingBindingFunction(outStmt, function (name) { return peekScope(name); }, componentRoot);
+              /*__WRNEXUS_DEV_END__*/
               return runStmt(outStmt, locals);
             } catch (error) {
               console.error(