consider Node's exitCode

lib/cli/run-helpers.js

@@ -25,7 +25,8 @@ exports.watchRun = watchRun;
  */
 const exitMochaLater = code => {
   process.on('exit', () => {
-    process.exitCode = Math.min(code, 255);
+    // keep exitCode set by Node (eg. uncaughtException)
+    process.exitCode = process.exitCode || Math.min(code, 255);
   });
 };
 
@@ -37,19 +38,15 @@ const exitMochaLater = code => {
  * @private
  */
 const exitMocha = code => {
-  const clampedCode = Math.min(code, 255);
+  process.exitCode = process.exitCode || Math.min(code, 255);
   let draining = 0;
 
-  // Eagerly set the process's exit code in case stream.write doesn't
-  // execute its callback before the process terminates.
-  process.exitCode = clampedCode;
-
   // flush output for Node.js Windows pipe bug
   // https://github.com/joyent/node/issues/6247 is just one bug example
   // https://github.com/visionmedia/mocha/issues/333 has a good discussion
   const done = () => {
     if (!draining--) {
-      process.exit(clampedCode);
+      process.exit();
     }
   };