THRIFT-4654 Minor fixes for dart 1 & 2 compatibility (backwards compatible) (#1617)

* THRIFT-4654 Minor fixes for dart 1 & 2 compatibility
* update dockerfiles for CI to test dart 2
* THRIFT-4654 update pubspec for tests
* THRIFT-4654 update dart test generator and readme to 1.24.3
* THRIFT-4654 update generated dart sdk to a range
* fix the dart tutorials
* Add .dart_tool directories to .gitignore
* THRIFT-4654: use dart 2.1.0 and fix cross test dart client timeout
diff --git a/tutorial/dart/client/web/index.html b/tutorial/dart/client/web/index.html
index 64b184e..9d36b43 100644
--- a/tutorial/dart/client/web/index.html
+++ b/tutorial/dart/client/web/index.html
@@ -24,8 +24,7 @@
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>Thrift Tutorial</title>
     <link rel="stylesheet" href="styles.css">
-    <script async src="client.dart" type="application/dart"></script>
-    <script async src="packages/browser/dart.js"></script>
+  <script async src="client.dart.js"></script>
 </head>
 
 <body>