In previous version of tsickle abstract class methods were materialized.
The change resulted in 6Kb savings in angular.io bundle.
This change also required the removal of `@private` and `@return` type
annotation as it is explicitly dissalowed by tsickle.
NOTE: removed casts in front of `makeDecorator` due to:
https://github.com/angular/devkit/issues/45
```
14938 Jul 19 13:16 0.b19e913fbdd6507d346b.chunk.js
1535 Jul 19 13:16 inline.d8e019ea3cfdd86c2bd0.bundle.js
589178 Jul 19 13:16 main.54c97bcb6f254776b678.bundle.js
34333 Jul 19 13:16 polyfills.4a3c9ca9481d53803157.bundle.js
14938 Jul 18 16:55 0.b19e913fbdd6507d346b.chunk.js
1535 Jul 18 16:55 inline.0c83abb44fad9a2768a7.bundle.js
582786 Jul 18 16:55 main.ea290db71b051813e156.bundle.js
34333 Jul 18 16:55 polyfills.4a3c9ca9481d53803157.bundle.js
main savings: 589178 - 582786 = 6,392
```
PR Close#18236
