Insufficient test coverage to ensure that compiler releases work with Angular

Angular is selling our users on Closure Compiler - as of Angular 4 we provide some support at http://github.com/angular/closure-demo
It's also a key part of our "develop like google does" story: http://g.co/ng/abc

However, we frequently find that closure releases are broken. Currently 20170409 is the latest release that works with Angular - however it won't work with Angular 5 because we need some recent fixes to node module resolution.

We should ensure that a green build of closure will work with Angular.

we should be able to copy what closure-library does to run their end-to-end tests externally, and use that to run the closure compiler IntegrationTest (currently it only runs in google-internal)

/cc @gregmagolan @MatrixFrog