[dartdev] Tweak compile exe depfile name

Use output name verbatim if provided, because that's what build-systems
like ninja expect.

[email protected]

Change-Id: Ia14add25cf200d7225852c1d10409230f2545a82
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/435681
Reviewed-by: Martin Kustermann <[email protected]>
Commit-Queue: Slava Egorov <[email protected]>

Author: mraleph

pkg/dart2native/lib/generate.dart

@@ -228,7 +228,7 @@ class _Generator {
       extraGenKernelOptions: [
         '--invocation-modes=compile',
         '--verbosity=$_verbosity',
-        if (_depFile != null) '--depfile-target=$_outputPath',
+        if (_depFile != null) '--depfile-target=${_outputFile ?? _outputPath}',
         ...?extraOptions,
       ],
       recordedUsagesFile: recordedUsagesFile,