Added a comment to clarify the interpretation of exit codes on windows

tools/testing/dart/test_runner.dart

Index: tools/testing/dart/test_runner.dart
diff --git a/tools/testing/dart/test_runner.dart b/tools/testing/dart/test_runner.dart
index b78d6d7cd22cad36c41d7a16debae632c2e641f0..a3e72c9712e24773ba8dad225a2e838761785d58 100644
--- a/tools/testing/dart/test_runner.dart
+++ b/tools/testing/dart/test_runner.dart
@@ -566,6 +566,12 @@ class CommandOutputImpl implements CommandOutput {
       if (exitCode == 3) {
         return !timedOut;
       }
+      // If a program receives an uncaught system exception, the program
+      // terminates with the exception code as exit code.
+      // The 0x3FFFFF00 mask here tries to determine if an exception indicates
+      // a crash of the program.
+      // System exception codes can be found in 'winnt.h', for example
+      // "#define STATUS_ACCESS_VIOLATION  ((DWORD) 0xC0000005)"
       return (!timedOut && (exitCode < 0) && ((0x3FFFFF00 & exitCode) == 0));
     }
     return !timedOut && ((exitCode < 0));