always throw when add missing helpers

[tanhauhau]

Q	A
Fixed Issues?	Fixes #10187
Patch: Bug Fix?	👍
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

The problem with current addHelper is that this line will get execute first, before knowing whether the helper existed:

// 2) will short circuit if `this.declarations[name]` exists
const declar = this.declarations[name];
if (declar) return t.cloneNode(declar);

// 1) will set this.declarations[name] no matter helper exists
const uid = (this.declarations[name] = this.scope.generateUidIdentifier(
   name,
));

so i added a helpers.ensure(name) before (1).

Since loadHelper is cached, it's okay to call it multiple times.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/11117/

[nicolo-ribaudo]

Related: #10200