dart2js: Add effects 'length' for arrays and 'other' for effects outside of the Dart execution model

[rakudrama]

We have effects for instance variables, static variables and indexing.
We should add separate effects for lengths and external effects (e.g. DOM accesses)

[sigmundch]

would the intent be to keep the two disjoint? that way native and js-interop methods have all "external" side-effects, but no internal side-effects?

[rakudrama]

Tracking lengths separately to other fields will help Dart-only array loops see that the length is not changed, allowing the length to be hoisted, better bounds check elimination, and the concurrent modification check to be removed from the for-in expansion for JSIndexables.

Generally native/interop methods can change everything, since they might call an escaped Dart closure, and I expect the union of effects over all escaped closures to be complete. Most native/interop methods don't call back, so we could annotate the most important native methods to have an effect without the effect being something that blocks optimizations on Dart fields.

native/interop methods can modify arrays, so I expect lengths and indexes to be seen as modified by native code by default.