Merge pull request #15964 from matt-kwong/fix-dart

Fix Dart interop tests by upgrading to Dart 2
diff --git a/templates/tools/dockerfile/interoptest/grpc_interop_dart/Dockerfile.template b/templates/tools/dockerfile/interoptest/grpc_interop_dart/Dockerfile.template
index 8b71716..4fb7b46 100644
--- a/templates/tools/dockerfile/interoptest/grpc_interop_dart/Dockerfile.template
+++ b/templates/tools/dockerfile/interoptest/grpc_interop_dart/Dockerfile.template
@@ -16,5 +16,8 @@
 
   FROM google/dart:latest
 
+  # Upgrade Dart to version 2.
+  RUN apt-get update && apt-get upgrade -y dart
+
   # Define the default command.
   CMD ["bash"]
diff --git a/tools/dockerfile/interoptest/grpc_interop_dart/Dockerfile b/tools/dockerfile/interoptest/grpc_interop_dart/Dockerfile
index d754996..bff20b5 100644
--- a/tools/dockerfile/interoptest/grpc_interop_dart/Dockerfile
+++ b/tools/dockerfile/interoptest/grpc_interop_dart/Dockerfile
@@ -14,5 +14,8 @@
 
 FROM google/dart:latest
 
+# Upgrade Dart to version 2.
+RUN apt-get update && apt-get upgrade -y dart
+
 # Define the default command.
 CMD ["bash"]