fix travis ci for pinned dart sdk version

.travis.yml

@@ -153,9 +153,6 @@ matrix:
       env: TARGET=csharp
       stage: main-test
     - os: linux
-      language: dart
-      dart:
-        - "2.8.4"
       jdk: openjdk8
       env: TARGET=dart
       stage: main-test

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

@@ -1,6 +1,7 @@
 #!/bin/bash
 
 set -euo pipefail
-
-dart --version
-command -v dart
+wget https://storage.googleapis.com/dart-archive/channels/stable/release/2.8.4/linux_packages/dart_2.8.4-1_amd64.debhttps://storage.googleapis.com/dart-archive/channels/stable/release/2.8.4/linux_packages/dart_2.8.4-1_amd64.deb
+sudo dpkg -i ./dart_2.8.4-1_amd64.deb
+sudo rm ./dart_2.8.4-1_amd64.deb
+sudo apt-get install -f

doc/dart-target.md

@@ -1,5 +1,7 @@
 # ANTLR4 Runtime for Dart
 
+Notice: Dart target may generate code incompatible with Dart 2.9 sound null safety. Please set the minimum SDK constraint to 2.8.4 or lower if such violation is found. Contributions are welcomed.
+
 ### First steps
 
 #### 1. Install ANTLR4