Fix all strong-mode warnings.

lib/src/util.dart

Index: lib/src/util.dart
diff --git a/lib/src/util.dart b/lib/src/util.dart
index 87060097d04a736f66d22d00f6c9984c35784eeb..a6b3ab4b5c8111911e08edcd9c18807c8bb7ffce 100644
--- a/lib/src/util.dart
+++ b/lib/src/util.dart
@@ -7,6 +7,8 @@ library matcher.util;
 import 'core_matchers.dart';
 import 'interfaces.dart';
 
+typedef bool _Predicate(value);
+
 /// A [Map] between whitespace characters and their escape sequences.
 const _escapeMap = const {
   '\n': r'\n',
@@ -38,7 +40,7 @@ void addStateInfo(Map matchState, Map values) {
 Matcher wrapMatcher(x) {
   if (x is Matcher) {
     return x;
-  } else if (x is Function) {
+  } else if (x is _Predicate) {
     return predicate(x);
   } else {
     return equals(x);