Remove forced override of package contents

Fixes #1155

src/Microsoft.CodeAnalysis.Testing/Microsoft.CodeAnalysis.Analyzer.Testing/ReferenceAssemblies.cs

@@ -460,16 +460,6 @@ namespace Microsoft.CodeAnalysis.Testing
                     }
                 }
 
-                // Prefer assemblies from the reference assembly package to ones otherwise provided
-                if (ReferenceAssemblyPackage is not null)
-                {
-                    var referenceAssemblies = new HashSet<string>(resolvedAssemblies.Where(resolved => resolved.StartsWith(referenceAssemblyInstalledPath!)));
-
-                    // Suppression due to https://github.com/dotnet/roslyn/issues/44735
-                    var referenceAssemblyNames = new HashSet<string>(referenceAssemblies.Select((Func<string, string>)Path.GetFileNameWithoutExtension!));
-                    resolvedAssemblies.RemoveWhere(resolved => referenceAssemblyNames.Contains(Path.GetFileNameWithoutExtension(resolved)) && !referenceAssemblies.Contains(resolved));
-                }
-
                 // Add the facade assemblies
                 if (ReferenceAssemblyPackage is not null)
                 {

tests/Microsoft.CodeAnalysis.Testing/Microsoft.CodeAnalysis.Analyzer.Testing.UnitTests/MetadataReferenceTests.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
+using System.Collections.Immutable;
 using System.Threading;
 using System.Threading.Tasks;
 using Xunit;
@@ -649,6 +650,37 @@ class TestClass {
             }.RunAsync();
         }
 
+        [Theory]
+        [InlineData("net462")]
+        [InlineData("net47")]
+        [InlineData("net471")]
+        [InlineData("net472")]
+        [InlineData("net48")]
+#if !(NETCOREAPP1_1 || NET46)
+        [InlineData("net6.0")]
+        [InlineData("net7.0")]
+        [InlineData("net8.0")]
+#endif
+        [InlineData("netstandard2.0")]
+        [InlineData("netstandard2.1")]
+        public async Task ResolveSystemCollectionsImmutable8(string targetFramework)
+        {
+            var testCode = @"
+using System.Collections.Frozen;
+
+class TestClass {
+  FrozenSet<int> TestMethod() => throw null;
+}
+";
+
+            await new CSharpTest()
+            {
+                TestCode = testCode,
+                ReferenceAssemblies = ReferenceAssembliesForTargetFramework(targetFramework)
+                    .AddPackages(ImmutableArray.Create(new PackageIdentity("System.Collections.Immutable", "8.0.0"))),
+            }.RunAsync();
+        }
+
         internal static ReferenceAssemblies ReferenceAssembliesForTargetFramework(string targetFramework)
         {
             return targetFramework switch