[dart2js] Support conversion of local function type variables from K to J model

This CL adds support for converting local function type variables from
the K model to the J model. The entity use for the J model type variables
isn't created before the closure classes and closure call methods have
been created. Therefore, the [ClosureData] is now registered
with the [JsToFrontendMap] object, once computed, allowing conversion of
local function type variables. Conversion of [BackendUsage] is moved
after closure creation because [RuntimeTypeUse] can refer to local
function type variables.

Closes #42088

Change-Id: Ifc00e69b5db0dd05710ea97017c41e8c7f5e520e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/153080
Reviewed-by: Mayank Patke <fishythefish@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>