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/test/server_test.dart

@@ -0,0 +1,40 @@
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:http/http.dart';
+import 'package:test/test.dart';
+
+// Manual test:
+// $ dart bin/server.dart
+// $ curl -X POST -d '{"by": 1}' -H "Content-Type: application/json" localhost:8080/
+
+void main() {
+  final port = '8080';
+  final host = 'http://0.0.0.0:$port';
+  late Process p;
+
+  setUp(() async {
+    p = await Process.start(
+      'dart',
+      ['run', 'bin/server.dart'],
+      environment: {'PORT': port},
+    );
+    // Wait for server to start and print to stdout.
+    await p.stdout.first;
+  });
+
+  tearDown(() => p.kill());
+
+  test('Increment', () async {
+    final response = await post(Uri.parse('$host/'), body: '{"by": 1}');
+    expect(response.statusCode, 200);
+    expect(response.body, '{"value":1}');
+  });
+
+  test('Get', () async {
+    final response = await get(Uri.parse('$host/'));
+    expect(response.statusCode, 200);
+    final resp = json.decode(response.body) as Map;
+    expect(resp.containsKey('value'), true);
+  });
+}