Add option to run `flutter-dev`

[brainwo]

flutter-dev is currently unusable in puro. Running it manually from puro env directory yield this:

$ flutter-dev
/home/brian/.local/share/puro/envs/default/flutter/bin/flutter-dev: line 69: /home/brian/.local/share/puro/envs/beta/flutter/bin/internal/shared.sh: No such file or directory

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flutter-dev is the command used to develop flutter_tools package—which is identical to flutter but doesn't use on-disk snapshot. https://github.com/flutter/flutter/blob/master/packages/flutter_tools/README.md#running-the-tool