Set 'BypassFrameworkInstallChecks' MSBuild property when running on Mono

'BypassFrameworkInstallChecks' allows builds to skip a particularly hacky check in the GetReferenceAssemblyPaths
task. This hack affects projects that target a .NET Framework version less than 4.0. Essentially, the hack tries
to load a certain assembly from the GAC to "guarantee" that .NET Framework 3.5 SP1 is installed (an old case added
specifically for WPF projects). However, we don't include that assembly in our Mono package. So, many Unity projects
end up failing the check because they often target much early .NET Framework versions. We can fix that by just
skipping the check, which isn't really necessary when we're running on Mono.

src/OmniSharp.Host/MSBuild/Discovery/Providers/StandAloneInstanceProvider.cs

@@ -30,19 +30,29 @@ public override ImmutableArray<MSBuildInstance> GetInstances()
 
             var propertyOverrides = ImmutableDictionary.CreateBuilder<string, string>(StringComparer.OrdinalIgnoreCase);
 
-            // To better support older versions of Mono that don't include
-            // MSBuild 15, we attempt to set property overrides to the locations
-            // of Mono's 'xbuild' and 'xbuild-frameworks' paths.
-            if (_allowMonoPaths && PlatformHelper.IsMono)
+            if (PlatformHelper.IsMono)
             {
-                if (TryGetMonoXBuildPath(out var xbuildPath))
+                // This disables a hack in the "GetReferenceAssemblyPaths" task which attempts
+                // guarantee that .NET Framework SP1 is installed when the target framework is
+                // .NET Framework, but the version is less than 4.0. The hack attempts to locate
+                // a particular assembly in the GAC as a "guarantee". However, we don't include that
+                // in our Mono package. So, we'll just bypass the check.
+                propertyOverrides.Add("BypassFrameworkInstallChecks", "true");
+
+                // To better support older versions of Mono that don't include
+                // MSBuild 15, we attempt to set property overrides to the locations
+                // of Mono's 'xbuild' and 'xbuild-frameworks' paths.
+                if (_allowMonoPaths)
                 {
-                    extensionsPath = xbuildPath;
-                }
-
-                if (TryGetMonoXBuildFrameworksPath(out var xbuildFrameworksPath))
-                {
-                    propertyOverrides.Add("TargetFrameworkRootPath", xbuildFrameworksPath);
+                    if (TryGetMonoXBuildPath(out var xbuildPath))
+                    {
+                        extensionsPath = xbuildPath;
+                    }
+
+                    if (TryGetMonoXBuildFrameworksPath(out var xbuildFrameworksPath))
+                    {
+                        propertyOverrides.Add("TargetFrameworkRootPath", xbuildFrameworksPath);
+                    }
                 }
             }
 

src/OmniSharp.MSBuild/ProjectFile/PropertyNames.cs

@@ -7,6 +7,7 @@ internal static class PropertyNames
         public const string AssemblyOriginatorKeyFile = nameof(AssemblyOriginatorKeyFile);
         public const string BuildProjectReferences = nameof(BuildProjectReferences);
         public const string BuildingInsideVisualStudio = nameof(BuildingInsideVisualStudio);
+        public const string BypassFrameworkInstallChecks = nameof(BypassFrameworkInstallChecks);
         public const string Configuration = nameof(Configuration);
         public const string CscToolExe = nameof(CscToolExe);
         public const string CscToolPath = nameof(CscToolPath);

src/OmniSharp.MSBuild/ProjectLoader.cs

@@ -52,6 +52,11 @@ private static Dictionary<string, string> CreateGlobalProperties(
             globalProperties.AddPropertyOverride(PropertyNames.Configuration, options.Configuration, propertyOverrides, logger);
             globalProperties.AddPropertyOverride(PropertyNames.Platform, options.Platform, propertyOverrides, logger);
 
+            if (propertyOverrides.TryGetValue(PropertyNames.BypassFrameworkInstallChecks, out var value))
+            {
+                globalProperties.Add(PropertyNames.BypassFrameworkInstallChecks, value);
+            }
+
             return globalProperties;
         }