Support posting of external data into dart as external uint8 arrays through the Dart API.

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 0161d793ec10549df923800bf14ddb6efe7b1cad..e508f09e1c312f497ff168c4f7aa944acc45394c 100755
--- a/runtime/include/dart_api.h
+++ b/runtime/include/dart_api.h
@@ -885,6 +885,7 @@ typedef struct _Dart_CObject {
     kString,
     kArray,
     kUint8Array,
+    kExternalUint8Array,
     kUnsupported,
     kNumberOfTypes
   } type;
@@ -903,6 +904,12 @@ typedef struct _Dart_CObject {
       int length;
       uint8_t* values;
     } as_byte_array;
+    struct {
+      int length;
+      uint8_t* data;
+      void* peer;
+      Dart_PeerFinalizer callback;
+    } as_external_byte_array;
   } value;
 } Dart_CObject;