Add support for dynamic imports

[bnf]

They are mapped to a promise that uses require()
in the background. Therefore "require" is added as dependency.

This PR raises abstract-syntax-tree dependnecy to at least 2.8 to
provide support for ImportExpression.

Also side-effect-middle test case is fixed, as that failed for me, due to another default ordering.

[emilos]

Awesome, thanks :) would be nice to double check cases like:

Promise.all([
  import('foo.js'),
  import('bar.js')
]

or chained imports, like:

import('foo.js').then(() => {
  return import('bar.js')
})

I'll just merge as is, good start :)

[bnf]

Thank you for the quick merge and the release – and this tool. :)

would be nice to double check cases like:

You are right, added in #55

[bnf]

Hey, I missed one point, regarding default exports. Sorry for that.

ES6 compatible code needs to use the .default property, which isn't available with AMD modules generated by @buxlabs/es6-to-amd, because the default export is mapped directly to a return (which is nice in general):

One might want to write the following invalid code, that currently works:

import('foo').then(foo => {
  // use default export, this is invalid but currently works
  console.log(foo);
});

To be ES2020 compatible that code needs to read:

import('foo').then(foo => {
  // Use default export of 'foo' module
  console.log(foo.default);
});

With the current implementation the correct code wouldn't work, as default wouldn't be available.
Therefore we could some automagic that wraps the module in an object with a default property, if it is the default export.
That is of course not always easy to guess, and gets a bit ugly. Please see #57 for a workaround that would/could work.