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/README.md

@@ -0,0 +1,49 @@
+A server app built using [Shelf](https://pub.dev/packages/shelf),
+configured to enable running with [Docker](https://www.docker.com/).
+
+This sample code handles HTTP GET requests to `/` and `/echo/<message>`
+
+# Running the sample
+
+## Running with the Dart SDK
+
+You can run the example with the [Dart SDK](https://dart.dev/get-dart)
+like this:
+
+```
+$ dart run bin/server.dart
+Server listening on port 8080
+```
+
+And then from a second terminal:
+```
+$ curl http://0.0.0.0:8080
+Hello, World!
+$ curl http://0.0.0.0:8080/echo/I_love_Dart
+I_love_Dart
+```
+
+## Running with Docker
+
+If you have [Docker Desktop](https://www.docker.com/get-started) installed, you
+can build and run with the `docker` command:
+
+```
+$ docker build . -t myserver
+$ docker run -it -p 8080:8080 myserver
+Server listening on port 8080
+```
+
+And then from a second terminal:
+```
+$ curl http://0.0.0.0:8080
+Hello, World!
+$ curl http://0.0.0.0:8080/echo/I_love_Dart
+I_love_Dart
+```
+
+You should see the logging printed in the first terminal:
+```
+2021-05-06T15:47:04.620417  0:00:00.000158 GET     [200] /
+2021-05-06T15:47:08.392928  0:00:00.001216 GET     [200] /echo/I_love_Dart
+```