dartdev: Add comment about analysisFinished.

Change-Id: Iefd2daa478e0b8426e9d8786c169fd8cbdd5e5c5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/405560
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Samuel Rawlins <[email protected]>

Author: srawlins

pkg/dartdev/lib/src/commands/analyze.dart

@@ -214,6 +214,8 @@ class AnalyzeCommand extends DartdevCommand {
       io.exit(_Result.crash.exitCode);
     });
 
+    // Note that we could be awaiting `null` here, if
+    // `AnalysisServer._analysisFinished` has not been initialized yet.
     await server.analysisFinished;
     analysisFinished = true;