Remove some Isolate::current_zone() calls, as it gets the zone from mutator thread not the current …

runtime/lib/string.cc

Index: runtime/lib/string.cc
diff --git a/runtime/lib/string.cc b/runtime/lib/string.cc
index 2eae245fa67c64d4fb35cd6812e0331d766daee6..62e5aacb324a1fb4488bb4e16cba183e2b9b2a27 100644
--- a/runtime/lib/string.cc
+++ b/runtime/lib/string.cc
@@ -20,7 +20,7 @@ DEFINE_NATIVE_ENTRY(String_fromEnvironment, 3) {
   GET_NATIVE_ARGUMENT(String, default_value, arguments->NativeArgAt(2));
   // Call the embedder to supply us with the environment.
   const String& env_value =
-      String::Handle(Api::CallEnvironmentCallback(isolate, name));
+      String::Handle(Api::CallEnvironmentCallback(thread, name));
   if (!env_value.IsNull()) {
     return Symbols::New(env_value);
   }