[Details] Loader and Registry interleaves

[caridy]

This is a follow up on the discussion related to PR #65

Participants: @dherman @caridy @guybedford

As part of #65, we have rearranged the internals of the loader, and its registry. As a result, we now have two separate entities with the following responsibilities:

• Loader provides a public api to interact with modules (via their names and their referrers), and it will be reflective to what you can do with the local loader from within a module.
• Registry provides low level api to interact with the registry of modules (via their keys), and it is bound to a loader instance.

But, there are two conflicting perspectives on the loader/registry responsibilities:

• perspective 1: the loader provides the high level interface and the registry provides the low level interface
• perspective 2: the loader provides loading pipeline and the registry stores the state
  it almost works for both perspectives

Few more notes:

• [[Registry]] internal slot is created and initialized in the constructor of the loader.
• It is not allowed to have multiple loaders sharing the same registries.
• a .registry getter in a loader instance will give you access to the corresponding registry (e.g.: let registry = System.loader.registry;)
• the registry has a backpointer to its loader ([[Loader]] internal slot) in case of side effects during the interaction with the registry (e.g.: System.loader.registry.provide() or System.loader.registry.error())

As a result of this separation, we have to separate the loader API:

Loader:

• Reflect.Loader.prototype.import(name[, referrer])
• Reflect.Loader.prototype.resolve(name[, referrer])
• Reflect.Loader.prototype.load(name[, referrer[, stage]])
• get Reflect.Loader.prototype.registry

note: load is now a primary use case to pre-fetch, and provision a module from a referrer.

Registry:

• Registry.prototype @@iterator
• Registry.prototype.lookup(key)
• Registry.prototype.install(key, module)
• Registry.prototype.uninstall(key)
• Registry.prototype.cancel(key)
• Registry.prototype.provide(key, stage, value)
• Registry.prototype.error(key, stage, value)

note: all these methods are low level, and driven by a key.

[caridy]

Solved by #65

[caridy]

Loader Objects: https://whatwg.github.io/loader/#loader-objects
Registry Objects: https://whatwg.github.io/loader/#registry