fix pubspec for publish

pubspec.yaml

@@ -7,6 +7,9 @@ authors:
 homepage: https://github.com/dart-flitter/flutter_webview_plugin
 version: 0.1.1
 
+environment:
+  sdk: ">=1.8.0 <2.0.0"
+
 flutter:
   plugin:
     androidPackage: com.flutter_webview_plugin