[driver] fix match state hand-off in matcher_util.dart (#3103)

Fixes https://github.com/flutter/flutter/issues/3094

packages/flutter_driver/lib/src/matcher_util.dart

@@ -6,11 +6,12 @@ import 'package:matcher/matcher.dart';
 
 /// Matches [value] against the [matcher].
 MatchResult match(dynamic value, Matcher matcher) {
-  if (matcher.matches(value, {})) {
+  Map<dynamic, dynamic> matchState = {};
+  if (matcher.matches(value, matchState)) {
     return new MatchResult._matched();
   } else {
     Description description =
-        matcher.describeMismatch(value, new _TextDescription(), {}, false);
+        matcher.describeMismatch(value, new _TextDescription(), matchState, false);
     return new MatchResult._mismatched(description.toString());
   }
 }

packages/flutter_driver/test/src/matcher_util_test.dart

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:test/test.dart';
+import 'package:flutter_driver/src/matcher_util.dart';
+
+void main() {
+  group('match', () {
+    test('matches', () {
+      _TestMatcher matcher = new _TestMatcher(1);
+      MatchResult ok = match(1, matcher);
+      expect(ok.hasMatched, isTrue);
+      expect(ok.mismatchDescription, isNull);
+      expect(matcher.matchState1 is Map<dynamic, dynamic>, isTrue);
+      expect(matcher.matchState2, isNull);
+    });
+
+    test('mismatches', () {
+      _TestMatcher matcher = new _TestMatcher(2);
+      MatchResult fail = match(1, matcher);
+      expect(fail.hasMatched, isFalse);
+      expect(fail.mismatchDescription, 'mismatch!');
+      expect(matcher.matchState1, matcher.matchState2);
+    });
+  });
+}
+
+class _TestMatcher extends Matcher {
+  int expected;
+  Map<dynamic, dynamic> matchState1;
+  Map<dynamic, dynamic> matchState2;
+
+  _TestMatcher(this.expected);
+
+  @override
+  bool matches(dynamic item, Map<dynamic, dynamic> matchState) {
+    matchState1 = matchState;
+    return item == expected;
+  }
+
+  @override
+  Description describeMismatch(dynamic item, Description mismatchDescription, Map<dynamic, dynamic> matchState, bool verbose) {
+    matchState2 = matchState;
+    mismatchDescription.add('mismatch!');
+    return mismatchDescription;
+  }
+
+  @override
+  Description describe(Description description) {
+    throw 'not implemented';
+  }
+}