Uses UglifyJS, and thus TypeScript needs to compile down to ES5, since
UglifyJS does not yet fully support ES6. Once all ES6 bugs in UglifyJS,
we should compile to ES6 again.
We now use webpack instead of SystemJS, effectively bundling modules
into one file (plus commons chunks) for every entry point. This results
in a much smaller extension size (almost half). Furthermore we use
yarn/npm even for extension run-time dependencies. This relieves us
from manually vendoring and building dependencies. It's also easier to
understand for new developers familiar with node.
