Support Dart2

[Mistic92]

Current code is not working with Dart2.

What I found as first is part with code

).then((String result) {

start working when replaced with

).then((dynamic result) {

But I don't have solution to fix problem with casting in "_setMethodCallHandlerListener"

type '_InternalLinkedHashMap' is not a subtype of type 'Map<String, List<double>>' where
  _InternalLinkedHashMap is from dart:collection
  Map is from dart:core
  String is from dart:core
  List is from dart:core
  double is from dart:core

[efortuna]

I made a fix for this issue, and submitted a PR, but in the meanwhile, you can just point to my fork: https://github.com/efortuna/flutterlocation

[Lyokone]

Pushed to master, thank you :)