Support 'dart.library.X' env variables in the VM.

runtime/vm/dart_api_impl.h

Index: runtime/vm/dart_api_impl.h
diff --git a/runtime/vm/dart_api_impl.h b/runtime/vm/dart_api_impl.h
index 9820417debabd26a98a6591b0aab8450d486dd3d..67c025725e9919691da3262a2404a10759ae72f4 100644
--- a/runtime/vm/dart_api_impl.h
+++ b/runtime/vm/dart_api_impl.h
@@ -260,12 +260,13 @@ class Api : AllStatic {
   static void SetWeakHandleReturnValue(NativeArguments* args,
                                        Dart_WeakPersistentHandle retval);
 
-  static RawString* CallEnvironmentCallback(Thread* thread,
-                                            const String& name);
+  static RawString* GetEnvironmentValue(Thread* thread, const String& name);
 
  private:
   static Dart_Handle InitNewHandle(Thread* thread, RawObject* raw);
 
+  static RawString* CallEnvironmentCallback(Thread* thread, const String& name);
+
   // Thread local key used by the API. Currently holds the current
   // ApiNativeScope if any.
   static ThreadLocalKey api_native_key_;