src/vs/editor/common/services/languageService.ts/LanguageSelection/onDidChange
class LanguageSelection:	public get onDidChange(): Event<string> {
		if (!this._listener) {
			this._listener = this._onDidChangeLanguages(() => this._evaluate());
		}
		if (!this._emitter) {
			this._emitter = new Emitter<string>({
				onDidRemoveLastListener: () => {
					this._dispose();
				}
			});
		}
		return this._emitter.event;
	}