add declare to class properties type annotations

[JLHwung]

Q	A
Fixed Issues?	Fixes e2e errors in #12244
License	MIT

Currently we have redundant initializer code unshifted in the constructors from type annotations. For example

babel/packages/babel-traverse/src/path/index.js

Lines 31 to 69 in 2782a54

	constructor(hub: HubInterface, parent: Object) {
	this.parent = parent;
	this.hub = hub;
	this.contexts = [];
	this.data = null;
	// this.shouldSkip = false; this.shouldStop = false; this.removed = false;
	this._traverseFlags = 0;
	this.state = null;
	this.opts = null;
	this.skipKeys = null;
	this.parentPath = null;
	this.context = null;
	this.container = null;
	this.listKey = null;
	this.key = null;
	this.node = null;
	this.scope = null;
	this.type = null;
	}
	parent: Object;
	hub: HubInterface;
	contexts: Array<TraversalContext>;
	data: Object;
	shouldSkip: boolean;
	shouldStop: boolean;
	removed: boolean;
	state: any;
	opts: ?Object;
	_traverseFlags: number;
	skipKeys: ?Object;
	parentPath: ?NodePath;
	context: TraversalContext;
	container: ?Object | Array<Object>;
	listKey: ?string;
	key: ?string;
	node: ?Object;
	scope: Scope;
	type: ?string;

is transformed as (See here for the full outputs)

constructor(hub, parent) {
    // ... ignored initializer code
    this.shouldSkip = void 0;
    this.shouldStop = void 0;
    this.removed = void 0;
    // ... ignored initializer code
    this._traverseFlags = void 0;
  }

this.shouldSkip = void 0; transformed from the property type annotations, is redundant because we have initialized it in constructors. What's more, it causes side effects if we skip proposal-class-properties when building Babel in #12244, in which it is transformed to

constructor(hub, parent) {
  this._traverseFlags = void 0;
}
removed;
get removed() {}
set removed(removed) {}

When these class property annotations were authored, flow didn't support declare class properties and they happen to be removed in the built artifacts via proposal-class-properties. This PR adds declare to these properties so it is future-safe (removed by flow-types) after we skip proposal-class-properties.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/31145/

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[nicolo-ribaudo]

I don't remember when I added that comment, but maybe this lets us do

babel/babel.config.js

Lines 107 to 108 in 2782a54

	// TODO: Use @babel/preset-flow when
	// https://github.com/babel/babel/issues/7233 is fixed

?