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/lib/dart/pubspec.yaml b/lib/dart/pubspec.yaml
index fdc4a0d..ded0269 100644
--- a/lib/dart/pubspec.yaml
+++ b/lib/dart/pubspec.yaml
@@ -24,20 +24,15 @@
 documentation: http://thrift.apache.org
 
 environment:
-  sdk: ">=1.13.0 <2.0.0"
+  sdk: ">=1.24.3 <3.0.0"
 
 dependencies:
+  dart2_constant: ^1.0.0
   fixnum: ^0.10.2
   http: ^0.11.3
   logging: ^0.11.0
 
 dev_dependencies:
-  # test
-  mockito: ^1.0.0
-  test: ^0.12.0
-
-  # dart_dev - https://github.com/Workiva/dart_dev
-  dart_dev: ^1.5.0
-  coverage: ^0.7.3
-  dart_style: ">=0.2.4 <0.3.0"
-  dartdoc: ^0.9.0
+  dart_dev: ^2.0.0
+  mockito: ">=2.2.2 <4.0.0"
+  test: ">=0.12.30 <2.0.0"