TypeScript compiler will now build to ES2015 code and modules. Babili is used to minify ES2015
code, providing an initial optimization that we couldn't previously get just from Uglify. Uses
Babel to convert ES2015 to UMD/ES5 code, and Uglify to minimize the output.
Note that the duplication in tsconfig.json files will be fixed in a followup CL now that we have tsconfig inheritance
BREAKING CHANGE: Angular 4 will support only TypeScript 2.1, so we no longer provide backwards compatibility to TS 1.8.