MochaAdapter: Fix TypeErorr due to arrow-function reporter

Follows-up 3708273, which revealed that the only reason
this code previously worked is that we compiled it to ES5 where
the arrow function became a normal function.

The CI build did not catch this because the integration test
for QUnit/Jasmine/Mocha is passing for QUnit and Jasmine and then
aborts silently without exit code due to a different bug, so it
has not actually been testing this code for a while, unfortunately.

I will address that in a separate commit soon.

Author: Krinkle

lib/adapters/MochaAdapter.js

@@ -6,23 +6,23 @@ module.exports = class MochaAdapter extends EventEmitter {
     super();
 
     this.mocha = mocha;
-    this.origReporter = mocha._reporter;
-
-    mocha.reporter((runner) => {
-      this.runner = runner;
-
-      // eslint-disable-next-line no-unused-vars
-      const origReporterInstance = new (this.origReporter.bind(this.mocha,
-        this.runner))();
-
-      runner.on('start', this.onStart.bind(this));
-      runner.on('suite', this.onSuite.bind(this));
-      runner.on('test', this.onTest.bind(this));
-      runner.on('pending', this.onPending.bind(this));
-      runner.on('fail', this.onFail.bind(this));
-      runner.on('test end', this.onTestEnd.bind(this));
-      runner.on('suite end', this.onSuiteEnd.bind(this));
-      runner.on('end', this.onEnd.bind(this));
+
+    // Mocha will instantiate the given function as a constructor
+    // even if you only need a callback.
+    // As such, it can't be an arrow function as those throw TypeError
+    // if instantiated.
+    const self = this;
+    mocha.reporter(function (runner) {
+      self.runner = runner;
+
+      runner.on('start', self.onStart.bind(self));
+      runner.on('suite', self.onSuite.bind(self));
+      runner.on('test', self.onTest.bind(self));
+      runner.on('pending', self.onPending.bind(self));
+      runner.on('fail', self.onFail.bind(self));
+      runner.on('test end', self.onTestEnd.bind(self));
+      runner.on('suite end', self.onSuiteEnd.bind(self));
+      runner.on('end', self.onEnd.bind(self));
     });
   }