pkg/(unit)test: include a copy of old matcher

lib/src/matcher/interfaces.dart

Index: lib/src/matcher/interfaces.dart
diff --git a/lib/src/matcher/interfaces.dart b/lib/src/matcher/interfaces.dart
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+++ b/lib/src/matcher/interfaces.dart
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+// Copyright (c) 2012, 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 unittest.matcher.interfaces;
+
+// To decouple the reporting of errors, and allow for extensibility of
+// matchers, we make use of some interfaces.
+
+/// Matchers build up their error messages by appending to
+/// Description objects. This interface is implemented by
+/// StringDescription. This interface is unlikely to need
+/// other implementations, but could be useful to replace in
+/// some cases - e.g. language conversion.
+abstract class Description {
+  int get length;
+
+  /// Change the value of the description.
+  Description replace(String text);
+
+  /// This is used to add arbitrary text to the description.
+  Description add(String text);
+
+  /// This is used to add a meaningful description of a value.
+  Description addDescriptionOf(value);
+
+  /// This is used to add a description of an [Iterable] [list],
+  /// with appropriate [start] and [end] markers and inter-element [separator].
+  Description addAll(String start, String separator, String end, Iterable list);
+}
+
+/// [expect] Matchers must implement/extend the Matcher class.
+/// The base Matcher class has a generic implementation of [describeMismatch]
+/// so this does not need to be provided unless a more clear description is
+/// required. The other two methods ([matches] and [describe])
+/// must always be provided as they are highly matcher-specific.
+abstract class Matcher {
+  const Matcher();
+
+  /// This does the matching of the actual vs expected values.
+  /// [item] is the actual value. [matchState] can be supplied
+  /// and may be used to add details about the mismatch that are too
+  /// costly to determine in [describeMismatch].
+  bool matches(item, Map matchState);
+
+  /// This builds a textual description of the matcher.
+  Description describe(Description description);
+
+  /// This builds a textual description of a specific mismatch. [item]
+  /// is the value that was tested by [matches]; [matchState] is
+  /// the [Map] that was passed to and supplemented by [matches]
+  /// with additional information about the mismatch, and [mismatchDescription]
+  /// is the [Description] that is being built to decribe the mismatch.
+  /// A few matchers make use of the [verbose] flag to provide detailed
+  /// information that is not typically included but can be of help in
+  /// diagnosing failures, such as stack traces.
+  Description describeMismatch(
+      item, Description mismatchDescription, Map matchState, bool verbose) =>
+      mismatchDescription;
+}