plugin: don't call process.emitWarning in cordova

Removes call to process.emitWarning in the cordova events code.
Logs with console.warn() instead of throwing an exception due to
the process object not existing in the cordova runtime.
Also adds test to avoid a regression.

tests/tests.js

@@ -505,6 +505,21 @@ exports.defineAutoTests = function () {
       nodejs.channel.post('test-echo', 'bar');
     });
 
+    it( 'should not throw error when going over MaxListeners' , function(done) {
+      let listenerNumberToTest = nodejs.channel.getMaxListeners()+1;
+      let numberCalled = 0;
+      for(let i=0;i<listenerNumberToTest;i++) {
+        nodejs.channel.addListener('echo', function(replyObj) {
+          numberCalled++;
+          if(numberCalled>=listenerNumberToTest) {
+            done();
+          }
+        });
+      }
+      expect(nodejs.channel.listenerCount('echo')).toBe(listenerNumberToTest);
+      nodejs.channel.post('test-echo', 'foo');
+    });
+
     afterEach( function(done) {
       nodejs.channel.removeAllListeners('echo');
       generalCleanUp('echo', done);

www/nodejs_events.js

@@ -278,15 +278,11 @@ function _addListener(target, type, listener, prepend) {
       m = $getMaxListeners(target);
       if (m && m > 0 && existing.length > m) {
         existing.warned = true;
-        const w = new Error('Possible EventEmitter memory leak detected. ' +
+        const warnMsg = 'Possible EventEmitter memory leak detected. ' +
                             `${existing.length} ${String(type)} listeners ` +
                             'added. Use emitter.setMaxListeners() to ' +
-                            'increase limit');
-        w.name = 'MaxListenersExceededWarning';
-        w.emitter = target;
-        w.type = type;
-        w.count = existing.length;
-        process.emitWarning(w);
+                            'increase limit';
+        console.warn(warnMsg);
       }
     }
   }