for-of on a non-iterable, non-arraylike should throw an error.

The utility JS code used when transpiling a for-of loop down to ES5
checks for the `Symbol.iterator` property first, then falls back to
iterating as if the thing to be iterated is an array-like-thing.
This can end up hiding a `TypedError` that should have been thrown.

Check to make sure the thing to iterate over has a `length` property
before just treating it like an array when transpiling a for-of loop.
If it doesn't, throw a `TypeError`.

PiperOrigin-RevId: 509969328

Author: brad4d

src/com/google/javascript/jscomp/js/es6/util/makeiterator.js

@@ -32,6 +32,11 @@ $jscomp.makeIterator = function(iterable) {
   // NOTE: Disabling typechecking because [] not allowed on @struct.
   var iteratorFunction = typeof Symbol != 'undefined' && Symbol.iterator &&
       (/** @type {?} */ (iterable)[Symbol.iterator]);
-  return iteratorFunction ? iteratorFunction.call(iterable) :
-      $jscomp.arrayIterator(/** @type {!Array} */ (iterable));
+  if (iteratorFunction) {
+    return iteratorFunction.call(iterable);
+  }
+  if (typeof iterable['length'] == 'number') {
+    return $jscomp.arrayIterator(/** @type {!Array} */ (iterable));
+  }
+  throw new Error(String(iterable) + ' is not an iterable or ArrayLike');
 };