Get rid of DART_SDK in pub

[munificent]

Pub uses that environment variable just for a couple of pub tests so that we can make sure we can parse VERSION files correctly.

If an end user sets it, though, it is very unlikely to do what they want. We should remove it entirely (and come up with a different way to test that parsing code), or at least rename it to something more specific.

[munificent]

Added Started label.

[munificent]

Added Fixed label.

[DanTup]

I'm using this variable in DartVS s a way to find the SDK; it was nice that you'd already been using it!

In the next version, we'll automatically download and unzip the latest SDK if it's not set, but this variable remains as a way to override that (eg. to use Dev Channel or an old version).

Are users going to be educated that they shouldn't ever set this; or does our use if it seem like a valid way of achieving what we want?

[DartBot]

This comment was originally written by @zoechi

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see also the discussion at SO http://stackoverflow.com/questions/26408682

[munificent]

This bug is specific to pub's use of DART_SDK. Pub used to require and read that even though pub itself is in the SDK. Now, pub just looks in the directory containing itself to find the SDK so it doesn't need this.

This doesn't mean users can't set a DART_SDK environment variable and use it. It's just that pub itself doesn't need it anymore.

[DartBot]

This issue has been moved to dart-lang/pub#715.