[dart2wasm] Introduce "load ids" to dart2wasm deferred loading.

Load ids provide a way to reduce the overhead of deferred loading. By
default deferred loading requires mapping `loadLibrary` calls to a list
of modules. This requires including (1) library uris, (2) prefix names
and (3) module names directly in the main module. With this loading
modules is easier as the loading function gets the exact filename.

Load ids provide an alternative approach where the compiler emits a
separate file mapping a load ID to the module set required for that ID.
An app could store this mapping on the server allowing the frontend to
include only the load ID in its request and have the server figure out
which modules to send back.

Internal serving infra uses module sets like this so this change allows
easier integration into that tooling. Dart2js already supports emitting
this deferred mapping JSON and internal infra is using that today.

Other changes include:
- Run the deferred loading transformer after TFA. This will exclude unused libraries in the resulting deferred loading map. Mark deferred helpers as entry points so that they don't get tree-shaken.
- Some changes to naming conventions of JS helpers.
- Use filename as module name to simplify JS helpers

Change-Id: I5e2f5e374de77c87095d08bfff6534506cb10652
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/454240
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Nate Biggs <natebiggs@google.com>