Consider using runtime checks instead of multi-targeting specific Unix distros in System.IO.Ports

[ViktorHofer]

System.IO.Ports currently targets Linux, OSX and FreeBSD even though their implementation is 95% identical. To reduce package size and build times runtime checks could be used for the 5% diverging functionality:

runtime/src/libraries/System.IO.Ports/src/System.IO.Ports.csproj

Lines 109 to 117 in 85aebc4

	<ItemGroup Condition="'$(TargetsLinux)' == 'true'">
	<Compile Include="System\IO\Ports\SerialPort.Linux.cs" />
	</ItemGroup>
	<ItemGroup Condition=" '$(TargetsOSX)' == 'true' ">
	<Compile Include="System\IO\Ports\SerialPort.OSX.cs" />
	</ItemGroup>
	<ItemGroup Condition=" '$(TargetsFreeBSD)' == 'true' ">
	<Compile Include="System\IO\Ports\SerialPort.FreeBSD.cs" />
	</ItemGroup>

As an example, when adding NetCoreAppCurrent configurations, the package grows disproportionally because of the amount of runtime specific assemblies.

cc @adamsitnik @carlossanlop @jozkee @jeffhandley @ericstj

Tagging subscribers to this area: @dotnet/area-system-io
See info in area-owners.md if you want to be subscribed.

Issue Details

---

System.IO.Ports currently targets Linux, OSX and FreeBSD even though their implementation is 95% identical. To reduce package size and build times runtime checks could be used for the 5% diverging functionality:

runtime/src/libraries/System.IO.Ports/src/System.IO.Ports.csproj

Lines 109 to 117 in 85aebc4

	<ItemGroup Condition="'$(TargetsLinux)' == 'true'">
	<Compile Include="System\IO\Ports\SerialPort.Linux.cs" />
	</ItemGroup>
	<ItemGroup Condition=" '$(TargetsOSX)' == 'true' ">
	<Compile Include="System\IO\Ports\SerialPort.OSX.cs" />
	</ItemGroup>
	<ItemGroup Condition=" '$(TargetsFreeBSD)' == 'true' ">
	<Compile Include="System\IO\Ports\SerialPort.FreeBSD.cs" />
	</ItemGroup>

.

cc @adamsitnik @carlossanlop @jozkee @jeffhandley @ericstj

Author:	ViktorHofer
Assignees:	-
Labels:	area-System.IO
Milestone:	-