Update build rules to point to Fuchsia SDK

rather than Flutter's buildroot

Change-Id: Ia6b3ac7fcec8d8276875b2adf8fc2cccc95b5dad
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/117770
Reviewed-by: Chinmay Garde <[email protected]>
Reviewed-by: Zach Anderson <[email protected]>
Commit-Queue: Zach Anderson <[email protected]>

Author: dnfield

runtime/vm/BUILD.gn

@@ -68,7 +68,7 @@ library_for_all_configs("libdart_vm") {
   extra_deps = [ "//third_party/icu" ]
   if (is_fuchsia) {
     if (using_fuchsia_sdk) {
-      extra_deps += [ "$fuchsia_sdk_root/pkg/lib/sys/cpp" ]
+      extra_deps += [ "$fuchsia_sdk_root/pkg:sys_cpp" ]
     } else {
       extra_deps += [
         # TODO(US-399): Remove time_service specific code when it is no longer