Add code sharing sample (#1444)

* code-sharing boilerplate

* initial commit of code-sharing logic

* documentation improvements

* added code-sharing to samples.yaml

* Updated counter UI to visually indicate communication with the server

* added code_sharing to CI

* CI changes to code_sharing

* fixed test with DI

* formatting

* added shared module to CI

* adds forgotten CI change

code_sharing/server/Dockerfile

@@ -0,0 +1,20 @@
+# Use latest stable channel SDK.
+FROM dart:stable AS build
+
+# Resolve app dependencies.
+WORKDIR /app
+COPY . .
+RUN dart pub get
+
+# AOT compile app.
+RUN dart compile exe bin/server.dart -o bin/server
+
+# Build minimal serving image from AOT-compiled `/server`
+# and the pre-built AOT-runtime in the `/runtime/` directory of the base image.
+FROM scratch
+COPY --from=build /runtime/ /
+COPY --from=build /app/bin/server /app/bin/
+
+# Start server.
+EXPOSE 8080
+CMD ["/app/bin/server"]