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..c78ea89ecaa5de9a4d059c2a2dd95e9b3d6f7591 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.
+  Api::Scope api_scope(isolate);
   Dart_EnvironmentCallback callback = isolate->environment_callback();
   if (callback != NULL) {
     Dart_Handle result = callback(Api::NewHandle(isolate, name.raw()));