Make Dart_IdentityEqual live up to its sepc and not leak boxing implementation detail. (Reland r275…

runtime/include/dart_api.h

Index: runtime/include/dart_api.h
diff --git a/runtime/include/dart_api.h b/runtime/include/dart_api.h
index 5377983b0c0d5aa4c8b81ce7fd8d1d7066f88b4e..9ed6d6998e56796ba0cefaf7cc7f2e9325f31bb7 100755
--- a/runtime/include/dart_api.h
+++ b/runtime/include/dart_api.h
@@ -365,7 +365,9 @@ DART_EXPORT Dart_Handle Dart_ToString(Dart_Handle object);
 /**
  * Checks to see if two handles refer to identically equal objects.
  *
- * This is equivalent to using the triple-equals (===) operator.
+ * If both handles refer to instances, this is equivalent to using the top-level
+ * function identical() from dart:core. Otherwise, returns whether the two
+ * argument handles refer to the same object.
  *
  * \param obj1 An object to be compared.
  * \param obj2 An object to be compared.