Allow sources to be provided as a file

[nex3]

Currently, all sources for a compilation must be provided as command-line arguments. However, if a module contains many sources, it can run up against command-line length limits. This could be avoided by providing an argument that takes a file that contains newline-separated source file names, like tar's --files-from. Bonus points if it's possible to provide files via stdin by passing --sources-from - or something like that.

[jmesserly]

Oh! so it turns out you can supply args via @file_path and all of those will be loaded:
https://github.com/dart-lang/dev_compiler/blob/617e3b69a61e6994bfc8eb9e2a807deda52cfd45/bin/dartdevc.dart#L143

I presume we have @jakemac53 to thank for this feature :)

[nex3]

Cool! I'll add that to #613.