Don't require eliminating certain type checks in unoptimized compilation.

Don't require eliminating certain type checks in unoptimized compilation.

The compiler relied on the graph builder to eliminate certain type
checks so that running without type check elimination required a
hack in the code generator to deal with the transition sentinel.

This CL makes type checks explicit in the AST and avoid inserting
them where they're not necessary (e.g. assigning null, sentinal values).

R=regis@google.com, srdjan@google.com

Committed: https://code.google.com/p/dart/source/detail?r=36734

[srdjan, 2014-05-27 12:50:08]

lgtm

[regis, 2014-05-27 16:17:54]

LGTM

[Florian Schneider, 2014-05-28 10:16:13]

Committed patchset #2 manually as r36734 (presubmit successful).