`_isSdkDir` has an incorrect check against `version` as a directory

[srawlins]

I can't find an installation of Dart where version is a directory, yet that is what is expected in _isSdkDir. This function used to (pre 0.1.0) just test whether a thing existed named version.

Indeed on my Mac Homebrew installation, version is a file:

$ file /usr/local/Cellar/dart/1.24.0-dev.6.9/libexec/version
/usr/local/Cellar/dart/1.24.0-dev.6.9/libexec/version: ASCII text
$ cat /usr/local/Cellar/dart/1.24.0-dev.6.9/libexec/version
1.24.0-dev.6.9

[devoncarew]

This is definitely a bug - likely related to my recent refactor. The good news is, clients should generally be using the getSdkPath() method instead.

[srawlins]

Whew, thanks!!