wip: update DateTimeProvider

Author: yuzurihaaa

lib/src/sdk/platforms/web/time_providers/web_time_provider.dart

@@ -51,4 +51,7 @@ class WebTimeProvider implements TimeProvider {
   /// for more information.
   @override
   Int64 get now => fromDOMHighResTimeStamp(window.performance.now());
+
+  @override
+  double get nowNanoseconds => window.performance.now();
 }

lib/src/sdk/time_providers/datetime_time_provider.dart

@@ -8,4 +8,7 @@ import 'time_provider.dart';
 class DateTimeTimeProvider implements TimeProvider {
   @override
   Int64 get now => Int64(DateTime.now().microsecondsSinceEpoch) * 1000;
+
+  @override
+  double get nowNanoseconds => DateTime.now().microsecondsSinceEpoch * 1000.0;
 }

lib/src/sdk/time_providers/time_provider.dart

@@ -15,5 +15,8 @@ abstract class TimeProvider {
   static const int nanosecondsPerMillisecond = 1000000;
 
   /// The current time, in nanoseconds since Unix Epoch.
+  @Deprecated('This getter will be removed in future without replacement.')
   Int64 get now;
+
+  double get nowNanoseconds;
 }