Make MS.Build InternalsVisibleTo MS.Build.Conversion.Core

Previous versions have shipped with InternalsVisibleTo set for the
Conversion.Core assembly. OSS version has never had that set which
causes some issues. For example, crashes trying to use the
ErrorUtilities VerifyThrow functionality which is all internal.

This tag corresponds to the version of MSBuild packaged with Visual S…

…tudio 2015 Update 3. The actual sources for that release are an internal repo at a state that corresponds to this commit.

This tag corresponds to the version of MSBuild packaged with Visual S…

…tudio 2015 Update 3 RC. The actual sources for that release are an internal repo at a state that corresponds to this commit.

Merge pull request dotnet#538 from radical/revert-osname

$(OS) should return only two values - `Windows_NT` and `Unix` .

This tag corresponds to the version of MSBuild packaged with Visual S…

…tudio 2015 Update 2. The actual sources for that release are an internal repo at a state that corresponds to this commit.

This tag corresponds to the version of MSBuild packaged with Visual S…

…tudio 2015 Update 2 RC. The actual sources for that release are an internal repo at a state that corresponds to this commit.

This tag corresponds to the version of MSBuild packaged with Visual S…

…tudio 2015 Update 2 CTP. The actual sources for that release are an internal repo at a state that corresponds to this commit.

Merge pull request dotnet#374 from cdmihai/osx-baseline

Osx Build (with xplat based dependencies)

Merge pull request dotnet#267 from AndyGerlicher/GetSDKReferenceFiles…

…PlatformFix

Fix GetSDKReferenceFiles folder compare bug