fix pub requirement

pubspec.yaml

@@ -8,7 +8,7 @@ homepage: https://github.com/dart-flitter/flutter_webview_plugin
 version: 0.1.6
 
 environment:
-  sdk: ">=1.8.0 <2.0.0"
+  sdk: ">=1.19.0 <2.0.0"
 
 flutter:
   plugin: