content test: Switch to the new checks package

Definitely spiffier than `matcher`.  Has useful static types,
and also a better API for producing clear diagnostics on failure.

Author: gnprice

README.md

@@ -94,6 +94,30 @@ See [upstream docs on `flutter test`][flutter-cookbook-unit-tests].
 
 ## Notes
 
+### Writing tests
+
+For unit tests, we use [the `checks` package][package-checks].
+This is a new package from the Dart team, currently in preview,
+which is [intended to replace][package-checks-migration] the
+old `matcher` package.
+
+This means that if you see example test code elsewhere that
+uses the `expect` function, we'd prefer to translate it into
+something in terms of `check`.  For help with that,
+see the [`package:checks` migration guide][package-checks-migration]
+and the package's [API docs][package-checks-api].
+
+Because `package:checks` is still in preview, the Dart team is
+open to feedback on the API to a degree that they won't be
+after it reaches 1.0.  So where we find rough edges, now is a
+good time to [report them as issues][dart-test-tracker].
+
+[package-checks]: https://pub.dev/packages/checks
+[package-checks-api]: https://pub.dev/documentation/checks/latest/checks/checks-library.html
+[package-checks-migration]: https://github.com/dart-lang/test/blob/master/pkgs/checks/doc/migrating_from_matcher.md
+[dart-test-tracker]: https://github.com/dart-lang/test/issues
+
+
 ### Editing API types
 
 We support Zulip Server 4.0 and later.  For API features added in

test/model/content_checks.dart

@@ -0,0 +1,41 @@
+import 'package:checks/checks.dart';
+import 'package:zulip/model/content.dart';
+
+extension ContentNodeChecks on Subject<ContentNode> {
+  void equalsNode(ContentNode expected) {
+    if (expected is ZulipContent) {
+      isA<ZulipContent>()
+        .nodes.deepEquals(expected.nodes.map(
+          (e) => it()..isA<BlockContentNode>().equalsNode(e)));
+        // A shame we need the dynamic `isA` there.  This
+        // version hits a runtime type error:
+        //   .nodes.deepEquals(expected.nodes.map(
+        //     (e) => it<BlockContentNode>()..equalsNode(e)));
+        // and with `it()` with no type argument, it doesn't type-check.
+        // TODO: report that as API feedback on deepEquals
+    } else if (expected is ParagraphNode) {
+      isA<ParagraphNode>()
+        ..wasImplicit.equals(expected.wasImplicit)
+        ..nodes.deepEquals(expected.nodes.map(
+          (e) => it()..isA<InlineContentNode>().equalsNode(e)));
+    } else {
+      // TODO handle remaining ContentNode subclasses that lack structural ==
+      equals(expected);
+    }
+  }
+}
+
+extension ZulipContentChecks on Subject<ZulipContent> {
+  Subject<List<BlockContentNode>> get nodes => has((n) => n.nodes, 'nodes');
+}
+
+extension ParagraphNodeChecks on Subject<ParagraphNode> {
+  Subject<bool> get wasImplicit => has((n) => n.wasImplicit, 'wasImplicit');
+  Subject<List<InlineContentNode>> get nodes => has((n) => n.nodes, 'nodes');
+}
+
+extension TextNodeChecks on Subject<TextNode> {
+  Subject<String> get text => has((n) => n.text, 'text');
+}
+
+// TODO write similar extensions for the rest of the content node classes

test/model/content_matchers.dart

@@ -1,17 +1,22 @@
-import 'package:test/test.dart';
+import 'package:checks/checks.dart';
+import 'package:test/scaffolding.dart';
 import 'package:zulip/model/content.dart';
 
-import 'content_matchers.dart';
+import 'content_checks.dart';
 
 void main() {
   test('parse a plain-text paragraph', () {
-    // TODO try to compact this further
-    expect(
-      parseContent('<p>hello world</p>'),
-      ZulipContentMatcher([
-        ParagraphNodeMatcher(
-          wasImplicit: equals(false),
-          nodes: [equals(const TextNode('hello world'))])
+    check(parseContent('<p>hello world</p>'))
+      .equalsNode(const ZulipContent(nodes: [
+        ParagraphNode(nodes: [TextNode('hello world')]),
+      ]));
+  });
+
+  test('parse two plain-text paragraphs', () {
+    check(parseContent('<p>hello</p><p>world</p>'))
+      .equalsNode(const ZulipContent(nodes: [
+        ParagraphNode(nodes: [TextNode('hello')]),
+        ParagraphNode(nodes: [TextNode('world')]),
       ]));
   });