Static interop should use compiler-dependent global context

[srujzs]

Currently, we use globalThis everywhere, but this is not necessarily available. Instead, we should have the compilers decide what to use for interop. For dart2js, this is likely self and for DDC, this is likely dart.global. Long-term, we should avoid making interop based on the global context unless requested and instead define a scope where we expect users to export their JS code to. We do need to be careful about the compilers shadowing names, however.

[rakudrama]

Is the global context something that is fixed, and can be captured for re-use?

Is dart2js allowed to reuse an access of the global context as an optimization?

[srujzs]

Talked offline about this but yes, we'd want dart2js to be able to cache this by marking this getter as "pure" through the JS foreign function's metadata. DDC naturally won't be able to do this, but that's okay, since that's the current behavior of interop and dart:html.