Reduce the bundle size with Google Closure Compiler

[vlukashov]

Investigate whether or not using the Google Closure Compiler reduces the minified gzipped bundle size for the core package. If so, switch from uglify-es onto google-closure-compiler for production builds.

DoD:

• build the core package with gcc and make sure all tests pass
• measure the difference in the prodution bundle size between gcc and uglify-es
• if gcc produces a smaller bundle, replace uglify-es with gcc
• in any case write a summary of the findings in this ticket

[abdonrd]

About reducing the package size... Could it make sense to remove some functionality? And allow to import it optionally. As for example the animation functionality.

[abdonrd]

Also about reduce size: https://github.com/vaadin/vaadin-router/pull/464