Added an "origins" section to the flat module `.metadata.json` files
that records where the original symbols was declared. This allows
correctly calculating relative path references recorded in metadata.
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.
