Fix const Clock() for Dart 1

pkgs/clock/lib/src/clock.dart

diff --git a/pkgs/clock/lib/src/clock.dart b/pkgs/clock/lib/src/clock.dart
index 0ff29bf..2984e30 100644
--- a/pkgs/clock/lib/src/clock.dart
+++ b/pkgs/clock/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);