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

runtime/vm/dart_api_impl.h

Index: runtime/vm/dart_api_impl.h
===================================================================
--- runtime/vm/dart_api_impl.h	(revision 36820)
+++ runtime/vm/dart_api_impl.h	(working copy)
@@ -194,14 +194,25 @@
   static Dart_Handle NewError(const char* format, ...) PRINTF_ATTRIBUTE(1, 2);
 
   // Gets a handle to Null.
-  static Dart_Handle Null();
+  static Dart_Handle Null() {
+    return null_handle_;
+  }
 
   // Gets a handle to True.
-  static Dart_Handle True();
+  static Dart_Handle True() {
+    return true_handle_;
+  }
 
-  // Gets a handle to False
-  static Dart_Handle False();
+  // Gets a handle to False.
+  static Dart_Handle False() {
+    return false_handle_;
+  }
 
+  // Gets a handle to EmptyString.
+  static Dart_Handle EmptyString() {
+    return empty_string_handle_;
+  }
+
   // Retrieves the top ApiLocalScope.
   static ApiLocalScope* TopScope(Isolate* isolate);
 
@@ -265,6 +276,7 @@
   static Dart_Handle true_handle_;
   static Dart_Handle false_handle_;
   static Dart_Handle null_handle_;
+  static Dart_Handle empty_string_handle_;
 
   friend class ApiNativeScope;
 };