pin dart version to 2.8.4 to get unit tests passing

.travis/before-install-linux-dart.sh

@@ -6,5 +6,5 @@ sudo apt-get -y install apt-transport-https
 sudo sh -c 'wget -qO- https://dl-ssl.google.com/linux/linux_signing_key.pub | apt-key add -'
 sudo sh -c 'wget -qO- https://storage.googleapis.com/download.dartlang.org/linux/debian/dart_stable.list > /etc/apt/sources.list.d/dart_stable.list'
 sudo apt-get -q update
-sudo apt-get -y install dart
+sudo apt-get -y install dart=2.8.4
 export PATH="$PATH:/usr/lib/dart/bin"

appveyor.yml

@@ -8,7 +8,7 @@ install:
   - git submodule update --init --recursive
   - cinst -y php --params "/InstallDir:C:\tools\php"
   - cinst -y composer
-  - cinst -y dart-sdk
+  - cinst -y dart-sdk --version=2.8.4
 build_script:
   - mvn -DskipTests install --batch-mode
   - msbuild /target:restore /target:rebuild /property:Configuration=Release /logger:"C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll" /verbosity:detailed runtime/CSharp/runtime/CSharp/Antlr4.dotnet.sln