Delegate to LoadStratgy for module information (#925)

- LoadStrategy now has the following:
  - `moduleForServerPath`
  - `serverPathForModule`
  - `serverPathForAppUri`
- Since there is a flavor of `LoadStrategy` for each compiler, we now have consistency with handling module names and no longer leak Google3 details externally (we'll have a Google3 specific LegacyStrategy internaly)
- Fix Frontend Server tests locally, in a follow up PR I'll enable them on Travis for the dev SDK
- Fix Readme tests due to `package:arg` update

Towards https://github.com/dart-lang/webdev/issues/910
Closes https://github.com/dart-lang/webdev/issues/917
22 files changed