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/shared/test/shared_test.dart

@@ -0,0 +1,17 @@
+import 'package:shared/shared.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test('Increment serialization', () {
+    expect(Increment(by: 3).toJson(), <String, dynamic>{'by': 3});
+  });
+  test('Increment derialization', () {
+    expect(Increment.fromJson(<String, dynamic>{'by': 5}), Increment(by: 5));
+  });
+  test('Count serialization', () {
+    expect(Count(3).toJson(), <String, dynamic>{'value': 3});
+  });
+  test('Count derialization', () {
+    expect(Count.fromJson(<String, dynamic>{'value': 5}), Count(5));
+  });
+}