Add setting to locate Dartium
Restrict 'launch' key bindings to 'dart_can_do_launch' context
Add third party lib
Add non-fatal ConfigError class
Add 'is_prefix' helper
Add helpers to find and control Dartium
Make project aware of runnable files
Implement smart build task

- Add a context to detect a state where a launch is possible
- Launch file/project

  If the active file is under 'web', start server and serve using
  Dartium.

  If the active file is under 'bin', run as a script.

The new smart build feature overrides the default build systems
only when the 'dart_can_do_launch' context is true. Otherwise,
the pre-existing build tasks will be fired as usual.
Merge pull request #229 from my-personal-forks/smart-build-clean

Smart build clean