Request: requesting a .packages file from http: import of an entry point

[sethladd]

If a Dart runtime fetches an entry point from an http: URI, according to DEP dart-archive/dart_enhancement_proposals#5 (detailed here: https://github.com/lrhn/dep-pkgspec/blob/master/DEP-pkgspec.md) , the Dart runtime should:

Look for a .packages file next to the program entry point (which can then not be given using a package: URI). For example running an application like:
dart http://example.com/smarty/main.dart
will cause the dart stand-alone VM to check for the existence of http://example.com/smarty/.packages, and if that URI returns a file, use the content for resolving package URIs in the application.

[iposva-google]

Fixed with https://codereview.chromium.org//1268313002