test: tests live binding on a let (demonstrates: #2431)

[naugtur]

Refs: #2431

This is the ModuleSource instance being produced in the test in SES:

ModuleSource {
  imports: [],
  exports: [ 'quuux' ],
  reexports: [],
  __syncModuleProgram__: "({   imports: $h‍_imports,   liveVar: $h‍_live,   onceVar: $h‍_once,   importMeta: $h‍____meta, }) => (function () { 'use strict';   $h‍_imports([]);   \n" +
    '    let $c‍_quuux=null;\n' +
    '    // Live binding of an exported variable.\n' +
    "$h‍_live.quuux($c‍_quuux);quuux='Hello World!';\n" +
    '})()\n' +
    '//# sourceURL=https://example.com/main.js\n',
  __liveExportMap__: [Object: null prototype] { quuux: [ 'quuux', true ] },
  __reexportMap__: [Object: null prototype] {},
  __fixedExportMap__: [Object: null prototype] {},
  __needsImportMeta__: false
}

Note how quuux='Hello World!' is not assigning to $c‍_quuux and the only reason it works is moduleLexicals being there.
Since there's no moduleLexicals in the bundler, that test is failing.

[kriskowal]

This is a known-limitation that we must at least document better.

[erights]

This is a known-limitation that we must at least document better.

I'm surprised. Not urgent, but I am curious:

I know we handle such live-binding assignments in general through the with-proxy mechanism, and the hidden callback functions registered by the transformed module code. But why doesn't it work in this case?

[kriskowal]

This is a known-limitation that we must at least document better.

I'm surprised. Not urgent, but I am curious:

I know we handle such live-binding assignments in general through the with-proxy mechanism, and the hidden callback functions registered by the transformed module code. But why doesn't it work in this case?

We don’t use a with block in the calling convention for bundles generated with our bundle.js. We also generally don’t use live bindings for “bootstrap bundles”. This is the tool we use to make the ses/dist/* scripts to replace Rollup, which we are going to use for more cases down the road, but it is a trusted and not a confined bundle format. We have not needed live binding support yet and I would prefer to avoid the with block indefinitely.

[naugtur]

Added a test documenting lack of live bindings support.