- Add Dart_EmptyString to return the canonical empty string similar to how

runtime/include/dart_api.h

Index: runtime/include/dart_api.h
===================================================================
--- runtime/include/dart_api.h	(revision 36820)
+++ runtime/include/dart_api.h	(working copy)
@@ -1262,13 +1262,18 @@
 /**
  * Returns the null object.
  *
- * Requires there to be a current isolate.
- *
  * \return A handle to the null object.
  */
 DART_EXPORT Dart_Handle Dart_Null();
 
 /**
+ * Returns the empty string object.
+ *
+ * \return A handle to the empty string object.
+ */
+DART_EXPORT Dart_Handle Dart_EmptyString();
+
+/**
  * Is this object null?
  */
 DART_EXPORT bool Dart_IsNull(Dart_Handle object);