Update .travis.yml

.travis.yml

@@ -7,6 +7,11 @@ dart: stable
 os: linux
 
 install:
+- apt-get install -y apt-transport-https
+- wget -qO- https://dl-ssl.google.com/linux/linux_signing_key.pub | sudo gpg --dearmor -o /usr/share/keyrings/dart.gpg
+- echo 'deb [signed-by=/usr/share/keyrings/dart.gpg arch=amd64] https://storage.googleapis.com/download.dartlang.org/linux/debian stable main' | sudo tee /etc/apt/sources.list.d/dart_stable.list
+- apt-get update
+- apt-get install -y dart
 - mkdir -p  ~/.pub-cache
 - |
   cat <<EOF > ~/.pub-cache/credentials.json