[DEGRADE] Object.assign polyfill isn't injected by ES2018 object rest props

[teppeis]

Input:

module.exports = () => {
  var {a, ...rest} = {a: 1, b: 2, c: 3};
  return a === 1 && rest.a === undefined && rest.b === 2 && rest.c === 3;
};

v20181008 injects Object.assign polyfill expectdly: output

v20181028 doesn't: output

In result, the compiled code throws TypeError: Object function Object() { [native code] } has no method 'assign'.

[brad4d]

I believe this is due to the transpilation of destructuring being moved after all checks, which includes it moving after the pass that injects polyfills.
It will be fixed automatically when we make the planned change to have the polyfills pass always inject all polyfills required by the difference between the input and output language modes, but that also requires changes to RemoveUnusedCode so it can safely remove the polyfills that aren't actually needed.

We should be able to fix this more quickly than that by changing the es6InjectRuntimeLibraries pass to explicitly inject the Object.assign polyfill.

[brad4d]

Internal Google bug created: b/119199280

[teppeis]

From v20190121, Object.assign is not injected for object spread properties too.

• v20190106 output
• v20190121 output

[teppeis]

Not yet fixed in v20191111: input / output

[teppeis]

Thanks!