refactor compiler's runtime libs

[jmesserly]

We inherited the runtime lib from dart2js (mostly, with some async support taken from the VM libs). Then we added some hand written JS. That unfortunately grew into a much larger set of required library code, was split up into JS "modules" and is now merged back into a single dart:_runtime library.

Things we should try and improve:
Restructure helpers libs that came from dart2js. The organization of these doesn't make sense in DDC. e.g. some libraries have very little of their original code left.

Restructure dart:_runtime. It has a somewhat tangled relationship with dart:core, as dart:core defines some important primitives (Object, Type). See #417 for one example, but looking through js_codegen, there are others.

Letting the compiler generate a single JS file for all of the transitive closure of dart:core might be a good start on the path towards sanity.

dart:_runtime is now ostensibly written in Dart, but that's a very shallow property. It doesn't use Dart for the most part. There are things like final fields that are initialized to a JS expression that makes an ES6 class. That should just be a Dart class.

Those are a few ideas to start, but I'm sure we'll find more over time.

[jmesserly]

This issue was moved to dart-lang/sdk#27316