Refactor DOM generator.

Refactor DOM generator.

Each target (approximately) is now represented by a 'System'.  This
should be helpful in merging in the dart:hmtl code - it can be another
System (or 2 - one for frog, one for VM).

There are more changes to come.  In particular, I want to make the
class/interface templates per-system.

Checkpoint


BUG=
TEST=

Committed: https://code.google.com/p/dart/source/detail?r=3172

[sra1, 2012-01-11 01:26:40]

tbr