Cancel stdout/stderr subscriptions once subprocesses are dead, add more debugging information in ca…

dart/tools/testing/dart/utils.dart

Index: dart/tools/testing/dart/utils.dart
diff --git a/dart/tools/testing/dart/utils.dart b/dart/tools/testing/dart/utils.dart
index b4455fa1a6042d7bc17e51a95b8d8c9eb934cc42..cb3f802d0b3ffbe1b83aeba4d2b1d75e02c179f9 100644
--- a/dart/tools/testing/dart/utils.dart
+++ b/dart/tools/testing/dart/utils.dart
@@ -10,6 +10,15 @@ import 'dart:convert';
 
 part 'legacy_path.dart';
 
+// This is the maximum time we expect stdout/stderr of subprocesses to deliver
+// data after we've got the exitCode.
+const Duration MAX_STDIO_DELAY = const Duration(seconds: 30);
+
+String MAX_STDIO_DELAY_PASSED_MESSAGE =
+"""Not waiting for stdout/stderr from subprocess anymore 
+($MAX_STDIO_DELAY passed). Please note that this could be an indicator 
+that there is a hanging process which we were unable to kill.""";
+
 class DebugLogger {
   static IOSink _sink;