Clean up code that locates SDK and SDK version.

This is prep work for SDK constraints. It moves the SDK
version-related code out of SdkSource (which will go away at some
point) into its own library.

I also removed the need for the DART_SDK env variable. It will
still be used if set (which the tests need), but if not set, Pub
will locate the SDK.
Clean up a few static warnings.

Review URL: https://codereview.chromium.org//11871028

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/path@17433 260f80e4-7a28-3924-810f-c04153c831b5
1 file changed
tree: b9279ae5c678f0e7e25f3d7e50d14ba7a60b8ee0
  1. lib/
  2. test/
  3. pubspec.yaml