dart2js cps: Change interceptor pipeline.

dart2js cps: Change interceptor pipeline.

Interceptor sharing/hoisting now takes places much later.

ShareInterceptors is now responsible for replacing interceptor calls
with constants, although determining if it CAN be replaced with a
constant is determined during type propagation where precise types are
available.

LetSinking has been removed; it was a leftover from when interceptors
were generated at definition-site.

As a follow-up on deleting LetSinking, creation expressions are now
considered effectively constant during assignment propagation. This is
safe because such expressions are not propagated into loops (this was
not always the case).

BUG=
R=kmillikin@google.com

Committed: https://github.com/dart-lang/sdk/commit/e5fc507c7873cea5d0b5db17a76b7bee3cddc463

[asgerf, 2015-09-04 10:56:03]

asgerf@google.com changed reviewers:
+ kmillikin@google.com, sra@google.com

[Kevin Millikin (Google), 2015-09-07 09:21:17]

lgtm

[asgerf, 2015-09-07 17:08:23]

Committed patchset #1 (id:1) manually as
e5fc507c7873cea5d0b5db17a76b7bee3cddc463 (presubmit successful).