[dart] Replace "system-temp" with "isolated-temp"

Bug: 25092
Change-Id: I32bcfd380c501ba6d5416014b6673cf08751125d
diff --git a/runtime/dart/meta/dart.cmx b/runtime/dart/meta/dart.cmx
index b103009..6799e09 100644
--- a/runtime/dart/meta/dart.cmx
+++ b/runtime/dart/meta/dart.cmx
@@ -5,7 +5,7 @@
     "sandbox": {
         "features": [
             "root-ssl-certificates",
-            "system-temp"
+            "isolated-temp"
         ],
         "services": [
             "fuchsia.deprecatedtimezone.Timezone",