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/tests.json b/test/tests.json
index 43d6ded..3381a1f 100644
--- a/test/tests.json
+++ b/test/tests.json
@@ -562,6 +562,7 @@
   {
     "name": "dart",
     "client": {
+      "timeout": 15,
       "transports": [
         "buffered",
         "framed",
@@ -577,6 +578,7 @@
       ],
       "command": [
         "dart",
+	"--enable-asserts",
         "test_client/bin/main.dart"
       ]
     },