Fix some error (#167)

* pubspec: Use double quotation instead of single.

* api: Sort import.

* api, event: Remove unused asset which never has null.

lib/src/api.dart

@@ -1,17 +1,16 @@
 import 'dart:async';
 import 'dart:io';
-import 'package:flutter/services.dart';
+
 import 'package:flutter/material.dart'
     show
         AlertDialog,
         BuildContext,
-        FlatButton,
         Navigator,
         Text,
         TextButton,
         Widget,
         showDialog;
-import 'package:flutter/services.dart' show MethodChannel;
+import 'package:flutter/services.dart' show MethodCall, MethodChannel;
 
 import 'actions.dart';
 import 'event.dart';
@@ -181,8 +180,7 @@ class FlutterCallkeep extends EventManager {
     if (resp != null) {
       var uuids = <String>[];
       resp.forEach((element) {
-        if (element != null && element is String)
-        uuids.add(element);
+        if (element is String) uuids.add(element);
       });
       return uuids;
     }

lib/src/event.dart

@@ -43,8 +43,6 @@ class EventManager {
   ///  -- do something here
   /// });
   void on<T extends EventType>(T eventType, void Function(T event) listener) {
-    assert(listener != null, 'Null listener');
-    assert(eventType != null, 'Null eventType');
     _addListener(eventType.runtimeType, listener);
   }
 
@@ -53,7 +51,6 @@ class EventManager {
   /// all the methods that call it enforce the types!!!!
   void _addListener(Type runtimeType, dynamic listener) {
     assert(listener != null, 'Null listener');
-    assert(runtimeType != null, 'Null runtimeType');
     try {
       var targets = listeners[runtimeType];
       if (targets == null) {

pubspec.yaml

@@ -4,8 +4,8 @@ version: 0.3.3
 homepage: https://github.com/flutter-webrtc/callkeep
 
 environment:
-  sdk: '>=2.12.0 <3.0.0'
-  flutter: '>=1.22.0'
+  sdk: ">=2.12.0 <3.0.0"
+  flutter: ">=1.22.0"
 
 dependencies:
   flutter: