Thought about output level.

All messages on StdErr should end up in logger
That is Message
Error: Message
    (Additional Information: B)
Should end up as Error Level: "Message", Warning Level: "    (Additional Information: B)"
NoWarn hides messages completely in IkvmImporterInternal.IssueMessage

src/IKVM.Tools.Runner/Importer/IkvmImporterLauncher.cs

@@ -299,7 +299,7 @@ public async Task<int> ExecuteAsync(IkvmImporterOptions options, CancellationTok
                 await LogEvent(IkvmToolDiagnosticEventLevel.Debug, "Executing {0} {1}", cli.TargetFilePath, cli.Arguments);
 
                 // send output to MSBuild
-                cli = cli.WithStandardErrorPipe(PipeTarget.ToDelegate(i => LogEvent(IkvmToolDiagnosticEventLevel.Information, i)));
+                cli = cli.WithStandardErrorPipe(PipeTarget.ToDelegate(i => LogEvent(IkvmToolDiagnosticEventLevel.Warning, i)));
                 cli = cli.WithStandardOutputPipe(PipeTarget.ToDelegate(i => LogEvent(IkvmToolDiagnosticEventLevel.Debug, i)));
 
                 // combine manual cancellation with timeout