Add Dart_Null, Dart_True, and Dart_False.

runtime/vm/dart_api_impl.h

Index: runtime/vm/dart_api_impl.h
===================================================================
--- runtime/vm/dart_api_impl.h	(revision 1035)
+++ runtime/vm/dart_api_impl.h	(working copy)
@@ -37,6 +37,15 @@
   // Generate a handle used to designate an error return.
   static Dart_Handle Error(const char* msg);
 
+  // Get a handle to Null.
+  static Dart_Handle Null();
+
+  // Get a handle to True.
+  static Dart_Handle True();
+
+  // Get a handle to False
+  static Dart_Handle False();
+
   // Allocate space in the local zone.
   static uword Allocate(intptr_t size);