Track external allocated memory for weak persistent handles.

runtime/bin/io_buffer.cc

Index: runtime/bin/io_buffer.cc
===================================================================
--- runtime/bin/io_buffer.cc	(revision 33235)
+++ runtime/bin/io_buffer.cc	(working copy)
@@ -12,7 +12,7 @@
   uint8_t* data = Allocate(size);
   Dart_Handle result = Dart_NewExternalTypedData(
       Dart_TypedData_kUint8, data, size);
-  Dart_NewWeakPersistentHandle(result, data, IOBuffer::Finalizer);
+  Dart_NewWeakPersistentHandle(result, data, size, IOBuffer::Finalizer);
 
   if (Dart_IsError(result)) {
     Free(data);