DAS plugins: Fix entrypoint deletion bug

I recently introduced this deletion logic but was deleting the wrong
file; this was deleting the `plugin.dart` file that `plugin.aot` is
compiled from. Over the weekend I found this issue with manual testing.
I've also verified this fix with manual testing.

Change-Id: I3f570368068117e6bb952ada0707a09ee97b765c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/506320
Auto-Submit: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

pkg/analysis_server/lib/src/plugin/plugin_manager.dart

@@ -540,16 +540,17 @@ class PluginManager {
 
     var stopwatch = Stopwatch()..start();
     var depfile = entrypoint.parent.getChildAssumingFile('depfile.txt');
-    if (entrypoint.exists) {
+    var aotSnapshotFile = entrypoint.parent.getChildAssumingFile('plugin.aot');
+    if (aotSnapshotFile.exists) {
       try {
         // Delete any existing AOT snapshot. On MacOS, sometimes this file
         // becomes quarantined due to a race condition with codesigning and
         // overwriting the file.
-        entrypoint.delete();
+        aotSnapshotFile.delete();
       } catch (e) {
         instrumentationService.logInfo(
-          'Could not delete existing AOT plugin entrypoint at "$entrypoint": '
-          '$e.',
+          'Could not delete existing AOT plugin entrypoint at '
+          '"$aotSnapshotFile": $e.',
         );
       }
     }