Surprising error when syncing a 'duplicate' of multiple blocks via realtime collaboration.

```
When syncing a ‘duplicate’ which involves multiple blocks we get a 
surprising error where the top level block’s id get looked up (in 
Blockly.Xml.domToBlock) and it’s not found.  However, the ‘duplicate’ 
still ends up working :-)
```

Original issue reported on code.google.com by `ma...@google.com` on 23 Jan 2014 at 7:09
