Incrementally recompute dominators when inlining.

Incrementally recompute dominators when inlining.

Before: we marked the entire graph's dominator tree invalid in case there
were multiple exits from an inlined function and recomputed dominators after
inlining.  Now: in case there are multiple exits we collect them in a fresh
join block and compute its immediate dominator as the nearest common
ancestor in the dominator tree over all predecessors.

Committed: https://code.google.com/p/dart/source/detail?r=21268

[Florian Schneider, 2013-04-11 11:30:18]

LGTM.

https://codereview.chromium.org/14067002/diff/1/runtime/vm/flow_graph_builder.cc
File runtime/vm/flow_graph_builder.cc (right):

https://codereview.chromium.org/14067002/diff/1/runtime/vm/flow_graph_builder...
runtime/vm/flow_graph_builder.cc:179: ASSERT(j >= 1);
I think you can even assert that there are always at least 1 block (graph entry)
in the join_dominators:

ASSERT(j > 1)

[Kevin Millikin (Google), 2013-04-11 11:44:47]

Committed patchset #1 manually as r21268 (presubmit successful).