Add mono-no-runtime-async test to verify Mono builds don't include runtime async assemblies.

mono-no-runtime-async/Program.cs

@@ -0,0 +1,49 @@
+// Mono does not support runtime async.
+// This test is ran against Mono builds to verify no assemblies require runtime async.
+
+using System.Reflection;
+using System.Reflection.Metadata;
+using System.Reflection.PortableExecutable;
+
+// Find the root of the .NET installation.
+// <dotnet-root>/shared/Microsoft.NETCore.App/<version>/System.Private.CoreLib.dll
+string corelib = typeof(object).Assembly.Location;
+string dotnetRoot = Path.GetFullPath(Path.Combine(Path.GetDirectoryName(corelib)!, "..", "..", ".."));
+if (!File.Exists(Path.Combine(dotnetRoot, "dotnet")))
+    throw new Exception($"Expected to find 'dotnet' in {dotnetRoot}");
+Console.WriteLine($"Searching for assemblies in {dotnetRoot}");
+
+var assemblies = Directory.EnumerateFiles(dotnetRoot, "*.dll", SearchOption.AllDirectories);
+int failureCount = 0;
+foreach (var assembly in assemblies)
+{
+    using var file = File.Open(assembly, FileMode.Open, FileAccess.Read);
+    using var reader = new PEReader(file);
+
+    if (!reader.HasMetadata)
+        continue;
+
+    var metadataReader = reader.GetMetadataReader();
+
+    foreach (var methodHandle in metadataReader.MethodDefinitions)
+    {
+        var method = metadataReader.GetMethodDefinition(methodHandle);
+
+        // Detect the use of runtime async through MethodImplAttributes.Async.
+        if ((method.ImplAttributes & MethodImplAttributes.Async) != 0)
+        {
+            Console.WriteLine($"FAIL: {assembly} has methods with MethodImplAttributes.Async");
+            failureCount++;
+            break;
+        }
+    }
+}
+
+if (failureCount > 0)
+{
+    Console.WriteLine($"FAIL: assemblies use runtime async. Mono does not support runtime async.");
+    return 1;
+}
+
+Console.WriteLine("PASS: no assemblies use runtime async.");
+return 0;

mono-no-runtime-async/mono-no-runtime-async.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>$(TestTargetFramework)</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>

mono-no-runtime-async/test.json

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+{
+  "name": "mono-no-runtime-async",
+  "enabled": true,
+  "requiresSdk": true,
+  "version": "11.0",
+  "versionSpecific": false,
+  "type": "bash",
+  "cleanup": true,
+  "skipWhen": [
+    "runtime=coreclr"
+  ],
+  "ignoredRIDs":[
+  ]
+}

mono-no-runtime-async/test.sh

@@ -0,0 +1,6 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+IFS=$'\n\t'
+
+dotnet run