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/test/dart/test_client/pubspec.yaml b/test/dart/test_client/pubspec.yaml
index d1642f8..113054a 100644
--- a/test/dart/test_client/pubspec.yaml
+++ b/test/dart/test_client/pubspec.yaml
@@ -22,10 +22,10 @@
 homepage: http://thrift.apache.org
 
 environment:
-  sdk: ">=1.13.0 <2.0.0"
+  sdk: ">=1.24.3 <3.0.0"
 
 dependencies:
-  args: ^0.13.0
+  args: ">=0.13.0 <2.0.0"
   http: ^0.11.0
   thrift:
     path: ../../../lib/dart
@@ -33,4 +33,4 @@
     path: ../gen-dart/thrift_test
 
 dev_dependencies:
-  test: "^0.12.0"
+  test: ">=0.12.30 <2.0.0"