Better error message.

Author: mbostock

src/generators/disposable.js

@@ -2,6 +2,9 @@ import that from "../that.js";
 
 export default function disposable(value, dispose) {
   let done = false;
+  if (typeof dispose !== "function") {
+    throw new Error("dispose is not a function");
+  }
   return {
     [Symbol.iterator]: that,
     next: () => done ? {done: true} : (done = true, {done: false, value}),