Fix fromEnvironment when called from isolates.

runtime/lib/bool.cc

Index: runtime/lib/bool.cc
diff --git a/runtime/lib/bool.cc b/runtime/lib/bool.cc
index d57574568dd127bfdfcd15ee92b40bbc6cd3f9ad..b4d69f536eebc34eff7a7fea9136f03c385402b6 100644
--- a/runtime/lib/bool.cc
+++ b/runtime/lib/bool.cc
@@ -21,6 +21,7 @@ DEFINE_NATIVE_ENTRY(Bool_fromEnvironment, 3) {
   GET_NON_NULL_NATIVE_ARGUMENT(String, name, arguments->NativeArgAt(1));
   GET_NATIVE_ARGUMENT(Bool, 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()));
@@ -40,6 +41,7 @@ DEFINE_NATIVE_ENTRY(Bool_fromEnvironment, 3) {
           String::Handle(String::New("Illegal environment value")));
     }
   }
+  Dart_ExitScope();
   return default_value.raw();
 }