Update to Dart 2.8.4 (#562)

diff --git a/.gitpod.Dockerfile b/.gitpod.Dockerfile
index 5d561d9..bea7ad2 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/2.8.3/linux_packages/dart_2.8.3-1_amd64.deb && \
-    sudo dpkg -i dart_2.8.3-1_amd64.deb && \
+    wget https://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 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 0c6eafa..8cb7dba 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
-  - "2.8.3"
+  - "2.8.4"
   - beta
 
 # Use Ubuntu 18.04 (Bionic Beaver) 
diff --git a/Dockerfile b/Dockerfile
index 22a7135..f14b588 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:2.8.3
+FROM google/dart:2.8.4
 
 # 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/pubspec.lock b/pubspec.lock
index d1368b9..2d477ec 100644
--- a/pubspec.lock
+++ b/pubspec.lock
@@ -653,4 +653,4 @@
     source: hosted
     version: "2.2.1"
 sdks:
-  dart: ">=2.8.3 <3.0.0"
+  dart: ">=2.8.4 <3.0.0"
diff --git a/pubspec.yaml b/pubspec.yaml
index 8db254d..62a8b4c 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -3,7 +3,7 @@
 
 environment:
   # Keep SDK aligned with .travis.yml and FROM line of Dockerfile
-  sdk: ^2.8.3
+  sdk: ^2.8.4
 
 dependencies:
   analyzer: ^0.39.0