Add Testing page to dart (#238)

Add `Testing` page to dart

---------

Signed-off-by: Sri Krishna <[email protected]>
Co-authored-by: Steve Ayers <[email protected]>
Signed-off-by: Sri Krishna <[email protected]>

docs/dart/testing.md

@@ -0,0 +1,95 @@
+---
+title: Testing
+sidebar_position: 6
+---
+
+Testing a client application can be a crucial part of ensuring its functionality
+and performance. When it comes to mobile/web applications, spinning up a full server
+to test against may not always be the best option. In the following sections, we
+will go through a couple of alternatives.
+
+## Widget Testing
+
+For widgets that interface with Connect, it will generally be preferable to mock your RPCs since a backend may not be
+available or desirable to access in a unit test. The easiest way to do this is via Connect's `FakeTransportBuilder`
+
+### Mocking Transports
+
+`FakeTransportBuilder` from `package:connectrpc/test.dart` creates an in-memory
+server with your own RPC implementations. It allows you to mock a backend to cover different behaviors in your widget.
+
+To illustrate, let's set up a very simple ELIZA service:
+
+```dart
+import 'package:connectrpc/connect.dart';
+//highlight-next-line
+import 'package:connectrpc/test.dart';
+//highlight-next-line
+import './gen/eliza.connect.spec.dart';
+
+final fakeTransport = FakeTransportBuilder().unary(
+  ElizaService.say,
+  (_, __) async {
+    return SayResponse(sentence: 'I feel happy.');
+  },
+).build();
+```
+
+It exposes four methods: `unary`, `server`, `client`, and `bidi`. All of them accept
+a `Spec` type. This can be obtained from the generated `*.connect.spec.dart` files.
+
+Under the hood, this fake transport behaves like a server.
+This means that you can access request headers, raise errors with details, and also
+mock streaming responses. Here is an example that raises an error on the fourth
+request:
+
+```dart
+var sentences = <String>[];
+final fakeTransport = FakeTransportBuilder().unary(
+  ElizaService.say,
+  (req, context) async {
+    sentences.add(req.sentence);
+    // highlight-next-line
+    if (sentences.length > 3) {
+      // highlight-next-line
+      throw ConnectException(
+        // highlight-next-line
+        Code.resourceExhausted,
+        // highlight-next-line
+        "I have no words anymore.",
+        // highlight-next-line
+      );
+      // highlight-next-line
+    }
+    // highlight-next-line
+    context.responseHeaders.add("foo", "bar");
+    return SayResponse(sentence: 'I feel happy.');
+  },
+).build();
+```
+
+You can also use expectations to assert that your client sends requests as expected:
+
+```dart
+final fakeTransport = FakeTransportBuilder().unary(
+  ElizaService.say,
+  (req, _) async {
+    // highlight-next-line
+    expect(req.sentence, "how do you feel?");
+    return SayResponse(sentence: 'I feel happy.');
+  },
+).build();
+```
+
+The `FakeTransportBuilder` can be chained to provide mocks for multiple methods:
+
+```dart
+final fakeTransport = FakeTransportBuilder().unary(
+  ElizaService.say,
+  (req, _) async {
+    return SayResponse(sentence: 'I feel happy.');
+  },
+).server(ElizaService.converse, (req, _) async* {
+  yield ConverseResponse();
+}).build();
+```