Fix support for BigInt literals when using the acorn plugin

[lukastaegert]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:
Closes #2626

Description

This should address the following issues when using the acorn-bigint plugin:

• bigint numbers wrongly being treated like "null" when checking conditionals for tree-shakeability on older environments
• bigints being passed to Math.pow when checking conditionals on newer environments

At the moment to ensure Node 6 compatibility, "**" will be transpiled to Math.pow which does not support bigints. At the same time, the acorn plugin will wrongly mark bigint literals to have the value "null" on older systems that do not support them. This solves those issues by treating bigint literals as "unknown" on all systems. Theoretically, it would also be possible to leave them as literals on newer environments and just fix the Math.pow issue but then this would make testing much harder and I was lazy.

[danielgindi]

Did not fix it for me (with 1.8.0)

Also adding

            acornInjectPlugins: [ require('acorn-bigint') ],

Does not help.

I'm testing with a simple "test.js" containing only const a = 10n.

[danielgindi]

Just saw this: acornjs/acorn#824 after debugging rollup+acorn.

I had to do a hack like this:

require('acorn') && (require.cache[require.resolve('acorn')].exports = require('rollup/node_modules/acorn'));

To make acornInjectPlugins work (forcing require('acorn') to retrieve the acorn instance from rollup). But please consider adding acorn-bigint` as a builtin plugin for rollup.

[lukastaegert]

Please consider adding acorn-bigint` as a builtin plugin for rollup

As it is stage 3 with a reasonable probability of becoming stage 4 soon, a PR would be accepted. Nevertheless I guess it would not hurt to show some moderate support for fixing this issue on acorn side on acornjs/acorn#824

At the moment I am not sure the maintainers recognize this as an architectural problem.