Add ReactDebugInstanceMap

[gaearon]

This PR is the first in a series of pull requests split from the new ReactPerf implementation in #6046.

Here, we introduce a new module called ReactDebugInstanceMap. It will be used in __DEV__ and, when the __PROFILE__ gate is added, in the __PROFILE__ builds. It will not be used in the production builds.

This module acts as a mapping between “debug IDs” (a new concept) and the internal instances. Not to be confused with the existing ReactInstanceMap that maps internal instances to public instances.

What are the “debug IDs” and why do we need them? Both the new ReactPerf and other consumers of the devtool API, such as React DevTools, need access to some data from the internal instances, such as the instance type display name, current props and children, and so on. Right now we let such tools access internal instances directly but this hurts our ability to refactor their implementations and burdens React DevTools with undesired implementation details such as having to support React ART in a special way.1

The purpose of adding ReactDebugInstanceMap is to only expose “debug IDs” instead of the internal instances to any devtools. In a future PR, whenever there is an event such as mounting, updating, or unmounting a component, we will emit an event in ReactDebugTool with the debug ID of the instance. We will also add an introspection API that lets any devtool pass an ID and get the information about the current children, props, state, display name, and so on, without exposing the internal instances.

ReactDebugInstanceMap has a concept of “registering” an instance. We plan to add the hooks that register an instance as soon as it is created, and unregister it during unmounting. It will only be possible to read information about the instance while it is still registered. If we add support for reparenting, we should be able to move the (un)registration code to different places in the component lifecycle without changing this code. The currently registered instances are held in the registeredInstancesByIDs dictionary.

There is also a reverse lookup dictionary called allInstancesToIDs which maps instances back to their IDs. It is implemented as a WeakMap so the keys are stable and we’re not holding onto the unregistered instances. If we’re not happy with WeakMap, one possible alternative would be to add a new field called _debugID to all the internal instances, but we don’t want to do this in production. Using WeakMap seems like a simpler solution here (and stable IDs are a nice bonus). This, however, means that the __DEV__ (and the future __PROFILE__) builds will only work in browsers that support our usage of WeakMap.