Add a [prints] matcher to the matcher library.

pkg/matcher/test/prints_matcher_test.dart

Index: pkg/matcher/test/prints_matcher_test.dart
diff --git a/pkg/matcher/test/prints_matcher_test.dart b/pkg/matcher/test/prints_matcher_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..183d4ef35b7a911aab37db8045c3c37efd4b8acb
--- /dev/null
+++ b/pkg/matcher/test/prints_matcher_test.dart
@@ -0,0 +1,104 @@
+// 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_matchers_test;
+
+import 'dart:async';
+
+import 'package:matcher/matcher.dart';
+import 'package:unittest/unittest.dart';
+
+import 'test_utils.dart';
+
+void main() {
+  initUtils();
+
+  group('synchronous', () {
+    test("passes with an expected print", () {
+      shouldPass(() => print("Hello, world!"), prints("Hello, world!\n"));
+    });
+
+    test("combines multiple prints", () {
+      shouldPass(() {
+        print("Hello");
+        print("World!");
+      }, prints("Hello\nWorld!\n"));
+    });
+
+    test("works with a Matcher", () {
+      shouldPass(() => print("Hello, world!"), prints(contains("Hello")));
+    });
+
+    test("describes a failure nicely", () {
+      shouldFail(() => print("Hello, world!"), prints("Goodbye, world!\n"),
+          "Expected: prints 'Goodbye, world!\\n' ''"
+          "  Actual: <Closure: () => dynamic> "
+          "   Which: printed 'Hello, world!\\n' ''"
+          "   Which: is different. "
+          "Expected: Goodbye, w ... "
+          "  Actual: Hello, wor ... "
+          "          ^ Differ at offset 0");
+    });
+
+    test("describes a failure with a non-descriptive Matcher nicely", () {
+      shouldFail(() => print("Hello, world!"), prints(contains("Goodbye")),
+          "Expected: prints contains 'Goodbye'"
+          "  Actual: <Closure: () => dynamic> "
+          "   Which: printed 'Hello, world!\\n' ''");
+    });
+
+    test("describes a failure with no text nicely", () {
+      shouldFail(() {}, prints(contains("Goodbye")),
+          "Expected: prints contains 'Goodbye'"
+          "  Actual: <Closure: () => dynamic> "
+          "   Which: printed nothing.");
+    });
+  });
+
+  group('asynchronous', () {
+    test("passes with an expected print", () {
+      shouldPass(() => new Future(() => print("Hello, world!")),
+          prints("Hello, world!\n"));
+    });
+
+    test("combines multiple prints", () {
+      shouldPass(() => new Future(() {
+        print("Hello");
+        print("World!");
+      }), prints("Hello\nWorld!\n"));
+    });
+
+    test("works with a Matcher", () {
+      shouldPass(() => new Future(() => print("Hello, world!")),
+          prints(contains("Hello")));
+    });
+
+    test("describes a failure nicely", () {
+      shouldFail(() => new Future(() => print("Hello, world!")),
+          prints("Goodbye, world!\n"),
+          "Expected: 'Goodbye, world!\\n' ''"
+          "  Actual: 'Hello, world!\\n' ''"
+          "   Which: is different. "
+          "Expected: Goodbye, w ... "
+          "  Actual: Hello, wor ... "
+          "          ^ Differ at offset 0",
+          isAsync: true);
+    });
+
+    test("describes a failure with a non-descriptive Matcher nicely", () {
+      shouldFail(() => new Future(() => print("Hello, world!")),
+          prints(contains("Goodbye")),
+          "Expected: contains 'Goodbye'"
+          "  Actual: 'Hello, world!\\n' ''",
+          isAsync: true);
+    });
+
+    test("describes a failure with no text nicely", () {
+      shouldFail(() => new Future.value(), prints(contains("Goodbye")),
+          "Expected: contains 'Goodbye'"
+          "  Actual: ''",
+          isAsync: true);
+    });
+  });
+}