[dart] Update zx_clock_get caller

We're changin zx_clock_get to return a zx_status_t. This change prepares
us for that change.

Test: No behavior change.
Change-Id: Ia172ccc55de78b888e353d127633b422bf7a4d1d
diff --git a/public/dart-pkg/zircon/sdk_ext/system.cc b/public/dart-pkg/zircon/sdk_ext/system.cc
index 854295a..2f62423 100644
--- a/public/dart-pkg/zircon/sdk_ext/system.cc
+++ b/public/dart-pkg/zircon/sdk_ext/system.cc
@@ -444,7 +444,9 @@
 }
 
 uint64_t System::ClockGet(uint32_t clock_id) {
-  return zx_clock_get(clock_id);
+  zx_time_t result = 0;
+  zx_clock_get_new(clock_id, &result);
+  return result;
 }
 
 // clang-format: off