resolvePromise method
Implementation
void resolvePromise(String promiseId, var returnValue) {
var returnValueNotNull = returnValue != null && returnValue != "" ? returnValue : "null";
String eventName = "webkitMessageHandler().resolvePromise";
String jsCall = "$eventName('$promiseId',$returnValueNotNull,null)";
controller.runJavaScript(jsCall);
}