Upgrade to Dart 2.9.1 (#579)

diff --git a/.gitpod.Dockerfile b/.gitpod.Dockerfile
index b70e3ad..3e0fe61 100644
--- a/.gitpod.Dockerfile
+++ b/.gitpod.Dockerfile
@@ -6,8 +6,8 @@
 # More information: https://www.gitpod.io/docs/config-docker/
 
 RUN sudo apt-get update && \
-    wget https://storage.googleapis.com/dart-archive/channels/stable/release/2.9.0/linux_packages/dart_2.9.0-1_amd64.deb && \
-    sudo dpkg -i dart_2.9.0-1_amd64.deb && \
+    wget https://storage.googleapis.com/dart-archive/channels/stable/release/2.9.1/linux_packages/dart_2.9.1-1_amd64.deb && \
+    sudo dpkg -i dart_2.9.1-1_amd64.deb && \
     sudo apt-get install -y protobuf-compiler redis && \
     sudo apt-get update && \
     echo "export PATH=\"\$PATH:/usr/lib/dart/bin:\$HOME/.pub-cache/bin\"" >> $HOME/.bashrc && \
diff --git a/.travis.yml b/.travis.yml
index 957c4c2..4bac4bf 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,7 +3,7 @@
   - gem install coveralls-lcov
 dart:
   # Keep this value aligned with min SDK value in pubspec.yaml and FROM line of Dockerfile
-  - "2.9.0"
+  - "2.9.1"
   - beta
 
 # Use Ubuntu 18.04 (Bionic Beaver) 
@@ -20,8 +20,8 @@
   - flutter doctor
   - flutter config --enable-web
   - flutter precache --web --no-android --no-ios --no-linux --no-windows --no-macos --no-fuchsia
-  - wget https://github.com/protocolbuffers/protobuf/releases/download/v3.11.4/protoc-3.11.4-linux-x86_64.zip
-  - unzip protoc-3.11.4-linux-x86_64.zip -d protoc
+  - wget https://github.com/protocolbuffers/protobuf/releases/download/v3.12.4/protoc-3.12.4-linux-x86_64.zip
+  - unzip protoc-3.12.4-linux-x86_64.zip -d protoc
   - export PATH=$PATH:$PWD/protoc/bin
   - pub global activate protoc_plugin
   - export PATH=$PATH:$HOME/.pub-cache/bin
diff --git a/Dockerfile b/Dockerfile
index 8fb1a82..a0f844d 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,5 +1,5 @@
 # Keep aligned with min SDK in pubspec.yaml and Dart test version in .travis.yml
-FROM google/dart:2.9.0
+FROM google/dart:2.9.1
 
 # The specific commit that dart-services should use. This should be kept
 # in sync with the flutter submodule in the dart-services repo.
diff --git a/cloud_run.Dockerfile b/cloud_run.Dockerfile
index 10aa2c4..1bc8011 100644
--- a/cloud_run.Dockerfile
+++ b/cloud_run.Dockerfile
@@ -1,5 +1,5 @@
 # Keep aligned with min SDK in pubspec.yaml and Dart test version in .travis.yml
-FROM google/dart:2.9.0
+FROM google/dart:2.9.1
 
 # The specific commit that dart-services should use. This should be kept
 # in sync with the flutter submodule in the dart-services repo.