[websocket] Implement HttpListenerContext.AcceptWebSocketAsync().
[websocket] Cleaner closing.
[websocket] Better hijacking.
[websocket] Use string.Equals instead of String.Compare
[websocket] Don't use string +=
Reimplement StreamWebSocket.
Fix some bugs.
Support CloseStatus
Support receiving frame larger than buffer.
Use do ... while (true)
Fix handling fragment and opcode.
Fix indent of author.
Remove checking Origin header.

RFC requires Origin header for browser client.
Non browser client may be omit Origin header.
[MWF] Fix opacity of form (#19856)
[xbuild] Add string to char[] argument conversion
[xbuild] Add missing apis
[mcs] Add handling for special case cref types/members
[System.Net.Http] Parsing productinfo like headers with missing version. Fixes #19881
[MS.Build] now ITaskItem impl. in BuildEngine4 behaves consistent with MS.Build.Tasks.

To be precise, now the implementation returns ItemSpec for ToString().
WriteLinesIntoFile task writes ITaskItem[] lines into a file, without calling
ItemSpec.
Maybe it is better to fix Tasks.dll but I'm unsure about xbuild compatibility.

This missing ToString() override caused target framework monikor output
to become invalid source file (i.e. it used to write ItemSpec as code(!) but
this ITaskItem impl. in BuildEngine4 just wrote its type name as .ToString()
result).
[MS.Build] add "OS" _undocumented_ predefined property.

It is used by Xamarin.Android to determine which compiler to use (csc or smcs)
and lack of this property simply caused Csc build task failure.