This requires delicate handling of type definitions which collide, because
we use TypeScript-provided lib.d.ts for --target=es5 and lib.es6.d.ts for
--target=es6.
We need to include our polyfill typings only in the --target=es5 case,
and the usages have to be consistent with lib.es6.d.ts.
Also starting with this change we now typecheck additional modules,
so this fixes a bunch of wrong typings which were never checked before.
Fixes#3178
StringMap really ought to always enforce a string
key, and also ought to have an index property.
This currently breaks too much code in Angular, so
should be fixed in a follow up Pull Request
separate from this.
But do it during the build process for cjs.
Right now we only need this when we transpile from ts
directly to es5. This is only the case in our
cis build, as for our browser build we only transpile
from ts to es6 via ts and then use traceur to do
the rest.