message: docs: add changelog for 5.2.1
diff: +<a name="5.2.1"></a>
  +## [5.2.1](https://github.com/angular/angular/compare/5.2.0...5.2.1) (2018-01-17)
  +
  +
  +### Bug Fixes
  +
  +* **animations:** fix increment/decrement aliases example ([#18323](https://github.com/angular/angular/issues/18323)) ([48c1898](https://github.com/angular/angular/commit/48c1898))
  +* **benchpress:** should still support selenium_webdriver < 3.6.0 ([#21477](https://github.com/angular/angular/issues/21477)) ([3c6a506](https://github.com/angular/angular/commit/3c6a506))
  +* **common:** set correct timezone for ISO8601 dates in Safari ([#21506](https://github.com/angular/angular/issues/21506)) ([8e9cd57](https://github.com/angular/angular/commit/8e9cd57)), closes [#21491](https://github.com/angular/angular/issues/21491)
  +* **compiler:** cache external reference resolution ([#21359](https://github.com/angular/angular/issues/21359)) ([c32e833](https://github.com/angular/angular/commit/c32e833))
  +* **compiler:** make `.ngsummary.json` files idempotent ([#21448](https://github.com/angular/angular/issues/21448)) ([a931a41](https://github.com/angular/angular/commit/a931a41))
  +* **core:** fix chained http call ([#20924](https://github.com/angular/angular/issues/20924)) ([54e7576](https://github.com/angular/angular/commit/54e7576)), closes [#20921](https://github.com/angular/angular/issues/20921)
  +* **language-service:** Clear caches when program changes ([#21337](https://github.com/angular/angular/issues/21337)) ([cc9419d](https://github.com/angular/angular/commit/cc9419d)), closes [#19405](https://github.com/angular/angular/issues/19405)
  +* **service-worker:** properly handle invalid hashes in all scenarios ([#21288](https://github.com/angular/angular/issues/21288)) ([51eb3d4](https://github.com/angular/angular/commit/51eb3d4))
  +
  +
  +### Features
  +
  +* **core:** add binding name to content changed error ([#20352](https://github.com/angular/angular/issues/20352)) ([4556532](https://github.com/angular/angular/commit/4556532))
  +* **forms:** handle string with and without line boundary on pattern validator ([#19256](https://github.com/angular/angular/issues/19256)) ([75f8522](https://github.com/angular/angular/commit/75f8522))
  +
  +
  +
  <a name="5.2.0"></a>
  # [5.2.0](https://github.com/angular/angular/compare/5.2.0-rc.0...5.2.0) (2018-01-10)
  