Fix const Clock() for Dart 1
diff --git a/lib/src/clock.dart b/lib/src/clock.dart
index 0ff29bf..2984e30 100644
--- a/lib/src/clock.dart
+++ b/lib/src/clock.dart
@@ -34,7 +34,7 @@
/// Creates a clock based on the given [currentTime], or on the system clock
/// by default.
- const Clock([DateTime currentTime()]) : _time = currentTime ?? systemTime;
+ const Clock([DateTime currentTime() = systemTime]) : _time = currentTime;
/// Creates [Clock] that always considers the current time to be [time].
Clock.fixed(DateTime time) : _time = (() => time);
