Don't treat the "cannot load analyzer" warning as an error in the tes…

…ts that treat all warnings as errors.

src/Assets/TestProjects/WatchNoDepsApp/WatchNoDepsApp.csproj

@@ -4,6 +4,15 @@
     <TargetFramework>$(CurrentTargetFramework)</TargetFramework>
     <OutputType>exe</OutputType>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+    <!--
+      Don't error when generators/analyzers can't be loaded.
+      This can occur when running tests against FullFramework MSBuild
+      if the build machine has an MSBuild install with an older version of Roslyn
+      than the generators in the SDK reference. We aren't testing the generators here
+      and this failure will occur more clearly in other places when it's
+      actually an important failure, so don't error out here.
+    -->
+    <WarningsNotAsErrors>CS9057</WarningsNotAsErrors>
   </PropertyGroup>
 
 </Project>

src/Tests/Microsoft.NET.Build.Tests/GivenThatWeWantToResolveConflicts.cs

@@ -227,7 +227,7 @@ public void FilesFromAspNetCoreSharedFrameworkAreNotIncluded()
             outputDirectory.Should().NotHaveFile("Microsoft.Extensions.DependencyInjection.Abstractions.dll");
         }
 
-        [Fact]
+        [CoreMSBuildOnlyFact]
         public void AnalyzersAreConflictResolved()
         {
             var testProject = new TestProject()
@@ -248,6 +248,14 @@ public void AnalyzersAreConflictResolved()
                     project.Root.Add(itemGroup);
                     itemGroup.Add(new XElement(ns + "EnableNETAnalyzers", "true"));
                     itemGroup.Add(new XElement(ns + "TreatWarningsAsErrors", "true"));
+
+                    // Don't error when generators/analyzers can't be loaded.
+                    // This can occur when running tests against FullFramework MSBuild
+                    // if the build machine has an MSBuild install with an older version of Roslyn
+                    // than the generators in the SDK reference. We aren't testing the generators here
+                    // and this failure will occur more clearly in other places when it's
+                    // actually an important failure, so don't error out here.
+                    itemGroup.Add(new XElement(ns + "WarningsNotAsErrors", "CS9057"));
                 });
 
             var buildCommand = new BuildCommand(testAsset);

src/Tests/Microsoft.NET.Publish.Tests/GivenThatWeWantToRunILLink.cs

@@ -1672,6 +1672,15 @@ private TestProject CreateTestProjectWithAnalysisWarnings(string targetFramework
                 TargetFrameworks = targetFramework,
                 IsExe = isExe
             };
+
+            // Don't error when generators/analyzers can't be loaded.
+            // This can occur when running tests against FullFramework MSBuild
+            // if the build machine has an MSBuild install with an older version of Roslyn
+            // than the generators in the SDK reference. We aren't testing the generators here
+            // and this failure will occur more clearly in other places when it's
+            // actually an important failure, so don't error out here.
+            // If we're actually testing that we build with no warnings, the test will still fail.
+            testProject.AdditionalProperties["WarningsNotAsErrors"] = "CS9057";
 
             testProject.SourceFiles[$"{projectName}.cs"] = @"
 using System;