Add a [prints] matcher to the matcher library.

pkg/matcher/lib/src/prints_matcher.dart

Index: pkg/matcher/lib/src/prints_matcher.dart
diff --git a/pkg/matcher/lib/src/prints_matcher.dart b/pkg/matcher/lib/src/prints_matcher.dart
new file mode 100644
index 0000000000000000000000000000000000000000..e3e3ccce7cca8be1d27a8a0a879d64e8dd8cd4ee
--- /dev/null
+++ b/pkg/matcher/lib/src/prints_matcher.dart
@@ -0,0 +1,73 @@
+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library matcher.prints_matcher;
+
+import 'dart:async';
+
+import 'description.dart';
+import 'expect.dart';
+import 'interfaces.dart';
+import 'future_matchers.dart';
+import 'util.dart';
+
+/// Matches a [Function] that prints text that matches [matcher].
+///
+/// [matcher] may be a String or a [Matcher].
+///
+/// If the function this runs against returns a [Future], all text printed by
+/// the function (using [Zone] scoping) until that Future completes will be

[Bob Nystrom, 2014/12/05 18:14:53]

"will be" -> "is".

[nweiz, 2014/12/09 01:01:01]

Done.

+/// matched.
+///
+/// This only tracks text printed using the [print] function.

[Bob Nystrom, 2014/12/05 18:14:53]

This means going between print(), stdout.write(), and/or console.write() can
break testable behavior. :(

[nweiz, 2014/12/09 01:01:01]

That's already true; anyone could be using a custom Zone.

+Matcher prints(matcher) => new _Prints(wrapMatcher(matcher));
+
+class _Prints extends Matcher {
+  final Matcher _matcher;
+
+  _Prints(this._matcher);
+
+  bool matches(item, Map matchState) {
+    if (item is! Function) return false;
+
+    var buffer = new StringBuffer();
+    var result = runZoned(item, zoneSpecification:
+        new ZoneSpecification(print: (_, __, ____, line) {
+      buffer.writeln(line);
+    }));
+
+    if (result is! Future) {
+      var actual = buffer.toString();
+      matchState['prints.actual'] = actual;
+      return _matcher.matches(actual, matchState);
+    }
+
+    return completes.matches(result.then(wrapAsync((_) {
+      expect(buffer.toString(), _matcher);
+    }, 'prints')), matchState);
+  }
+
+  Description describe(Description description) =>
+      description.add('prints ').addDescriptionOf(_matcher);
+
+  Description describeMismatch(item, Description description, Map matchState,
+      bool verbose) {
+    var actual = matchState.remove('prints.actual');
+    if (actual == null) return description;
+    if (actual.isEmpty) return description.add("printed nothing.");
+
+    description.add('printed ').addDescriptionOf(actual);
+
+    // Create a new description for the matcher because at least
+    // [_StringEqualsMatcher] replaces the previous contents of the description.
+    var innerMismatch = _matcher.describeMismatch(
+        actual, new StringDescription(), matchState, verbose).toString();
+
+    if (innerMismatch.isNotEmpty) {
+      description.add('\n   Which: ').add(innerMismatch.toString());
+    }
+
+    return description;
+  }
+}