Fix Dart 1.17 strong-mode warnings.

test/utils.dart

Index: test/utils.dart
diff --git a/test/utils.dart b/test/utils.dart
index 7b65bcb70e09fc2df0b14b163f54eeff39d84ac3..50e106bd57a8acf6bba5f38d9e0b9a37a4270f01 100644
--- a/test/utils.dart
+++ b/test/utils.dart
@@ -11,10 +11,13 @@ import "package:test/test.dart";
 /// A zero-millisecond timer should wait until after all microtasks.
 Future flushMicrotasks() => new Future.delayed(Duration.ZERO);
 
+typedef void OptionalArgAction([a, b]);
+
 /// A generic unreachable callback function.
 ///
 /// Returns a function that fails the test if it is ever called.
-unreachable(String name) => ([a, b]) => fail("Unreachable: $name");
+OptionalArgAction unreachable(String name) =>
+        ([a, b]) => fail("Unreachable: $name");
 
 // TODO(nweiz): Use the version of this in test when test#418 is fixed.
 /// A matcher that runs a callback in its own zone and asserts that that zone