Dynamic import() is not blockable as a global

[natevw]

Evel'ing this successfully returns a Promise:

import('test')

Testing in Chrome 80, the promise gets rejected with an Error: Cannot import module from an inactive browsing context. but it's possible that may vary depending on JS engine and/or iframe sandbox attribute support.

Dynamic import() works via a "function-like" keyword, not an actual call to a built-in function (e.g. it's like if or catch, rather than like eval or isNaN). We are unable to block keywords through shadowing (i.e. Function('import', "") throws just like var yield = return; would).

Even the Agoric/realms-shim ends up having to munge incoming source on account of this very issue: https://github.com/Agoric/realms-shim/blob/025d975da12c8033022c271e5d99a3810066dfa4/src/sourceParser.js#L31