In JS, bogus noImplicitAny error on some this-property assignments

[sandersn]

Discovered while looking at the puppeteer user test:

class EmulationManager {
  constructor() {
    this._emulatingMobile = false;
  }

  /**
   * @param {boolean} viewport
   */
  emulateViewport(viewport) {
    if (this._emulatingMobile !== viewport)
        return true;
    this._emulatingMobile = viewport;
  }
}

Expected behavior:
No error with noImplicitAny, and this._emulatingMobile: boolean.

Actual behavior:
Error on this._emulatingMobile = false, "implicitly has type 'any' because it is referenced directly or indirectly in its own initialiser, and this.emulatingMobile: any.