message: docs: add changelog for 5.2.0-beta.0
diff: +<a name="5.2.0-beta.0"></a>
  +# [5.2.0-beta.0](https://github.com/angular/angular/compare/5.1.0...5.2.0-beta.0) (2017-12-13)
  +
  +
  +### Features
  +
  +* **animations:** re-introduce support for transition matching functions ([#20723](https://github.com/angular/angular/issues/20723)) ([590d93b](https://github.com/angular/angular/commit/590d93b)), closes [#18959](https://github.com/angular/angular/issues/18959)
  +* **compiler:** add a pseudo $any() function to disable type checking ([#20876](https://github.com/angular/angular/issues/20876)) ([70cd124](https://github.com/angular/angular/commit/70cd124))
  +* **compiler:** narrow types of expressions used in *ngIf ([#20702](https://github.com/angular/angular/issues/20702)) ([e7d9cb3](https://github.com/angular/angular/commit/e7d9cb3))
  +* **core:** add source to `StaticInjectorError` message ([#20817](https://github.com/angular/angular/issues/20817)) ([b7738e1](https://github.com/angular/angular/commit/b7738e1)), closes [#19302](https://github.com/angular/angular/issues/19302)
  +* **forms:** allow nulls on setAsyncValidators ([#20327](https://github.com/angular/angular/issues/20327)) ([d41d2c4](https://github.com/angular/angular/commit/d41d2c4)), closes [#20296](https://github.com/angular/angular/issues/20296)
  +
  +
  +
  <a name="5.1.1"></a>
  ## [5.1.1](https://github.com/angular/angular/compare/5.1.0...5.1.1) (2017-12-13)
  