Squashed 'src/corefx/System.Collections.Immutable/' content from commit ec7c8de

git-subtree-dir: src/corefx/System.Collections.Immutable
git-subtree-split: ec7c8de32f81a4cb5fe71cba7fd0778fa778e3f3
Merge commit '24f7f426dc27c333e0657a421e8c574227dd8d40' as 'src/corefx/System.Collections.Immutable'
Squashed 'src/corefx/System.Reflection.Metadata/' content from commit 9d82bc0

git-subtree-dir: src/corefx/System.Reflection.Metadata
git-subtree-split: 9d82bc006a7c28dea43b05c9726b724679bc55b4
Merge commit '8781dc802afcfb7490a9f26ff3d030525d8c2213' as 'src/corefx/System.Reflection.Metadata'
[Build] Integrate more class library into the build (esp. System, System.Core)