DummyCompiler class), and make sure to call it everywhere it is
needed (in particular for attribute replacements).

Emit a new opcode, "mode", with an argument "html" or "xml", depending
on the value of the xml argument to the constructor.  (This requires a
matching checkin of TALInterpreter.)  (This comment was mistakenly
added to the previous checkin of DummyCompiler.py.)

Don't append to self.program directly; always call self.emit().