MSBuild repo doesn't build without 3.5 SP1 Targeting Pack

[KirillOsenkov]

On a clean Windows machine with VS 2019 16.6 where 3.5 SP1 targeting pack was never installed, this very MSBuild repo fails to build with:

C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\MSBuild\Current\
Bin\Microsoft.Common.CurrentVersion.targets(1177,5): error MSB3645: .NET Framew
ork v3.5 Service Pack 1 was not found. In order to target ".NETFramework,Versio
n=v3.5", .NET Framework v3.5 Service Pack 1 or later must be installed. [C:\MSB
uild\src\MSBuildTaskHost\MSBuildTaskHost.csproj]

When trying to add https://www.nuget.org/packages/Microsoft.NETFramework.ReferenceAssemblies/1.0.0 I've found out that it omits net35 from the list of frameworks it supports. See dotnet/installer#2022 for details.

Fortunately, our friend @jnm2 went out of his way to fill this gap and created this awesome package: https://www.nuget.org/packages/jnm2.ReferenceAssemblies.net35

However I found that it still doesn't work because of this check that checks for the "sentinel" System.Data.Entity.dll being installed in the GAC:

msbuild/src/Tasks/GetReferenceAssemblyPaths.cs

Lines 160 to 183 in e8c17c1

	#if FEATURE_GAC
	// This is a very specific "hack" to ensure that when we're targeting certain .NET Framework versions that
	// WPF gets to rely on .NET FX 3.5 SP1 being installed on the build machine.
	// This only needs to occur when we are targeting a .NET FX prior to v4.0
	if (!BypassFrameworkInstallChecks && moniker.Identifier.Equals(".NETFramework", StringComparison.OrdinalIgnoreCase) &&
	moniker.Version.Major < 4)
	{
	// We have not got a value for whether or not the 35 sentinel assembly has been found
	if (!s_net35SP1SentinelAssemblyFound.HasValue)
	{
	// get an assemblyname from the string representation of the sentinel assembly name
	var sentinelAssemblyName = new AssemblyNameExtension(NET35SP1SentinelAssemblyName);
	string path = GlobalAssemblyCache.GetLocation(sentinelAssemblyName, SystemProcessorArchitecture.MSIL, runtimeVersion => "v2.0.50727", new Version("2.0.57027"), false, new FileExists(p => FileUtilities.FileExistsNoThrow(p)), GlobalAssemblyCache.pathFromFusionName, GlobalAssemblyCache.gacEnumerator, false);
	s_net35SP1SentinelAssemblyFound = !String.IsNullOrEmpty(path);
	}
	// We did not find the SP1 sentinel assembly in the GAC. Therefore we must assume that SP1 isn't installed
	if (!s_net35SP1SentinelAssemblyFound.Value)
	{
	Log.LogErrorWithCodeFromResources("GetReferenceAssemblyPaths.NETFX35SP1NotIntstalled", TargetFrameworkMoniker);
	}
	}
	#endif

[KirillOsenkov]

Aha, I see they left us an escape hatch! Setting BypassFrameworkInstallChecks lets us bypass that entire block!