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

tools/testing/dart/test_runner.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * Classes and methods for executing tests.
  *
  * This module includes:
  * - Managing parallel execution of tests, including timeout checks.
  * - Evaluating the output of each test as pass/fail/crash/timeout.
@@ skipping 548 matching lines @@
   bool get hasCrashed {
     // The Java dartc runner and dart2js exits with code 253 in case
     // of unhandled exceptions.
     if (exitCode == 253) return true;
     if (io.Platform.operatingSystem == 'windows') {
       // The VM uses std::abort to terminate on asserts.
       // std::abort terminates with exit code 3 on Windows.
       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));
   }
 
   bool get hasTimedOut => timedOut;
 
   bool get didFail {
     return (exitCode != 0 && !hasCrashed);
   }
@@ skipping 1168 matching lines @@
         // the developer doesn't waste his or her time trying to fix a bunch of
         // tests that appear to be broken but were actually just flakes that
         // didn't get retried because there had already been one failure.
         bool allowRetry = _MAX_FAILED_NO_RETRY > _progress.numFailedTests;
         new RunningProcess(test, allowRetry, this).start();
       }
       _numProcesses++;
     }
   }
 }