Log Message:
mono: Update 6.12.0.199

Changelog:
mono-6.12.0.199

Bump nuget.exe to v6.6.1

mono-6.12.0.198

Fix xar url again

(cherry picked from commit 3005442)

mono-6.12.0.190

Change download URL for PCL reference assemblies

mono-6.12.0.188

Bump NuGetSdkResolver in msbuild

mono-6.12.0.185

Backport fixes for sharing wrappers when type attributes are involved (…

…#21537)

* [wasm] Fix the handling of i8/u8 in get_wrapper_shared_type_full (). (#19859)

Previously, these were returned verbatim, which caused sharing issues when the \ 
type had
attributes.

Fixes #19841.

* [aot] Fix the handling of r4/r8 parameter types with attributes during generic \ 
sharing. (#20217)

The attributes need to be ignored as with the other types, otherwise gsharedvt \ 
wrappers for signatures with
parameters like double f = default will not be found.

Fixes #20195.

mono-6.12.0.184

Backporting dotnet/runtime#59861 (#21532)

Fixes dotnet/runtime#72181

mono-6.12.0.183

Backport dotnet/runtime#71436 (#21519)

Backport dotnet/runtime#71436
Backport of #21516 to 2020-02