dart2js CPS: Do not transform unreachable branches.

dart2js CPS: Do not transform unreachable branches.

After type propagation, avoid transforming unreachable branches by
explicitly replacing them with Unreachable.  Also use the abstract boolean
value rather than the concrete constant value for branches, just as was
already done for type tests and casts.

R=asgerf@google.com
BUG=

Committed: https://github.com/dart-lang/sdk/commit/1e4469c14464f7dcadcf3d9f92bac6f4a2c96fc6

[Kevin Millikin (Google), 2015-08-28 09:12:12]

Ideally we'd be able to assert that a continuation body is Unreachable exactly
when the type propagation analysis computed that it was unreachable, but that
doesn't quite work for some cases.

This solution relies on the order of transformation --- if an unreachable
continuation has already been visited and transformed before the branch, then
analysis results from it can have polluted the analysis for other, reachable
continuations.

I'm a little worried about mixing analysis and transformation.  It seems like we
could transform based on our initial analysis, then reanalyze and conclude
something worse which would have invalidated our earlier transformation.  It's
hard to imagine this happening and being unsafe unless there's a bug in the
initial analysis, but it's still strange to think about.

[asgerf, 2015-08-28 10:18:24]

LGTM.

[Kevin Millikin (Google), 2015-08-28 11:10:50]

Committed patchset #1 (id:1) manually as
1e4469c14464f7dcadcf3d9f92bac6f4a2c96fc6 (presubmit successful).