I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[X] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/angular/angular/blob/master/CONTRIBUTING.md#question

Current behavior

if you have a tsconfig that extends another tsconfig that angularCompilerOptions or not extended and merged.

Expected behavior

angularCompilerOptions are merged and extended similar to what happens for compilerOptions

What is the motivation / use case for changing the behavior?

This is useful if you have a lot of tsconfigs and you want to have a base tsconfig. Case in point is for example the material2 repo. That every component has a tsconfig https://github.com/angular/material2/blob/master/src/lib/button/tsconfig-build.json

And they need to copy and paste all of the angularCompilerOptions over and over again

Environment

Angular version: 5.2.X