Add support for medium integers to the native message format

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 9d3fe1f4ef76d38c857deb15b0346562f3d69d0f..47da873c5e014d7adb1428abc99257d0a6aab0a0 100755
--- a/runtime/include/dart_api.h
+++ b/runtime/include/dart_api.h
@@ -548,6 +548,7 @@ struct Dart_CObject {
     kNull = 0,
     kBool,
     kInt32,
+    kInt64,
     kDouble,
     kString,
     kArray,
@@ -558,6 +559,7 @@ struct Dart_CObject {
   union {
     bool as_bool;
     int32_t as_int32;
+    int64_t as_int64;
     double as_double;
     char* as_string;
     struct {
@@ -599,6 +601,7 @@ DART_EXPORT bool Dart_PostCObject(Dart_Port port_id, Dart_CObject* message);
  * data references from the message are allocated by the caller and
  * will be reclaimed when returning to it.
  */
+
 typedef void (*Dart_NativeMessageHandler)(Dart_Port dest_port_id,
                                           Dart_Port reply_port_id,
                                           Dart_CObject* message);