[Feature Request] Code Search for pub

Something like https://cs.chromium.org/ would be really nice to have for pub.

It would help Flutter contributors understand the impact of (potentially) breaking changes better. It would also help the community be able to find example implementations of classes/methods/etc. they're interested in.

It would be important that this functionality allow searching __all source files__ contained in a package, not just the public portions of the library. It would be _nice_ if it could include searching of any non-Dart source as well (such as Java/Kotlin/ObjC/Swift files in Flutter plugins, but potentially other langauges as Dart FFI becomes a reality).

/cc @goderbauer @hixie