Fix fromEnvironment when called from isolates.

runtime/lib/string.cc

Index: runtime/lib/string.cc
diff --git a/runtime/lib/string.cc b/runtime/lib/string.cc
index 47a82ed60692bcb932bf92dfc39d335b6720b5a3..96a557414917ed9e449df54f8c46c239600b8276 100644
--- a/runtime/lib/string.cc
+++ b/runtime/lib/string.cc
@@ -19,6 +19,7 @@ DEFINE_NATIVE_ENTRY(String_fromEnvironment, 3) {
   GET_NON_NULL_NATIVE_ARGUMENT(String, name, arguments->NativeArgAt(1));
   GET_NATIVE_ARGUMENT(String, default_value, arguments->NativeArgAt(2));
   // Call the embedder to supply us with the environment.
+  Dart_EnterScope();
   Dart_EnvironmentCallback callback = isolate->environment_callback();
   if (callback != NULL) {
     Dart_Handle result = callback(Api::NewHandle(isolate, name.raw()));
@@ -37,6 +38,7 @@ DEFINE_NATIVE_ENTRY(String_fromEnvironment, 3) {
           String::Handle(String::New("Illegal environment value")));
     }
   }
+  Dart_ExitScope();
   return default_value.raw();
 }