[fuchsia] Add wrapper for zx_clock_get_monotonic.

shell/platform/fuchsia/dart-pkg/zircon/lib/src/system.dart

@@ -189,6 +189,7 @@ class System extends NativeFieldWrapperClass2 {
   static MapResult vmoMap(Handle vmo) native 'System_VmoMap';
 
   // Time operations.
+  static int clockGetMonotonic() native 'System_ClockGetMonotonic';
   static int clockGet(int clockId) native 'System_ClockGet';
 
   // TODO(edcoyne): Remove this, it is required to safely do an API transition across repos.

shell/platform/fuchsia/dart-pkg/zircon/sdk_ext/system.cc

@@ -459,6 +459,10 @@ Dart_Handle System::VmoMap(fml::RefPtr<Handle> vmo) {
   return ConstructDartObject(kMapResult, ToDart(ZX_OK), object);
 }
 
+uint64_t System::ClockGetMonotonic() {
+  return zx_clock_get_monotonic();
+}
+
 uint64_t System::ClockGet(uint32_t clock_id) {
   zx_time_t result = 0;
   zx_clock_get(clock_id, &result);

shell/platform/fuchsia/dart-pkg/zircon/sdk_ext/system.h

@@ -61,6 +61,8 @@ class System : public fml::RefCountedThreadSafe<System>,
 
   static Dart_Handle VmoMap(fml::RefPtr<Handle> vmo);
 
+  static uint64_t ClockGetMonotonic();
+
   static uint64_t ClockGet(uint32_t clock_id);
 
   static void RegisterNatives(tonic::DartLibraryNatives* natives);