Support sending and receiving Capability objects from C code.

runtime/include/dart_native_api.h

Index: runtime/include/dart_native_api.h
diff --git a/runtime/include/dart_native_api.h b/runtime/include/dart_native_api.h
index 2176aadeabb9ab5a013ec541c44be2c29ba9d313..ea7900df6be69603e2e3ad1d83f928e60496ff6c 100644
--- a/runtime/include/dart_native_api.h
+++ b/runtime/include/dart_native_api.h
@@ -40,6 +40,7 @@ typedef enum {
   Dart_CObject_kTypedData,
   Dart_CObject_kExternalTypedData,
   Dart_CObject_kSendPort,
+  Dart_CObject_kCapability,
   Dart_CObject_kUnsupported,
   Dart_CObject_kNumberOfTypes
 } Dart_CObject_Type;
@@ -62,6 +63,9 @@ typedef struct _Dart_CObject {
       Dart_Port origin_id;
     } as_send_port;
     struct {
+      int64_t id;
+    } as_capability;
+    struct {
       intptr_t length;
       struct _Dart_CObject** values;
     } as_array;