dart2js cps: Add helpers for common IR manipulation.

dart2js cps: Add helpers for common IR manipulation.

This adds four helpers:
- InteriorExpression: remove, insertAbove, and insertBelow.
- Primitive: destroy.

As discussed elsewhere, using such helpers is a double-edged sword.
They make the code easier to read and write, but it's easy to end up
with helpers that look like they work for a particular purpose, but
actually don't because of slight variations in different use-cases.

These methods have a couple of pitfalls, like forgetting to unlink
references after a call to `remove`, or accidentally using the body
of an orphaned node. But these are generally caught quickly in
checked-mode so, it seems like it will be worth it.

BUG=
R=kmillikin@google.com

Committed: https://github.com/dart-lang/sdk/commit/85f25787834ec7b5c09ceb7f9d5a27196c0ad11b

Reverted: https://github.com/dart-lang/sdk/commit/e4536dd274adf9cd3c8835ecfce98d4b75b5990e

Committed: https://github.com/dart-lang/sdk/commit/808f5e2228df9c41a909fcc31c481334a14a1d2b

[asgerf, 2015-09-28 12:26:09]

Patchset #2 (id:20001) has been deleted

[asgerf, 2015-09-28 12:40:54]

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

[Kevin Millikin (Google), 2015-09-28 13:25:02]

LGTM!  What's not to like, except for the backwards OO Yoda-talk that comes from
putting the verb after its object?

[asgerf, 2015-09-29 09:26:24]

Committed patchset #3 (id:60001) manually as
85f25787834ec7b5c09ceb7f9d5a27196c0ad11b (presubmit successful).

[asgerf, 2015-09-29 11:06:49]

I really should have tested in checked-mode after going on about how quickly
these bugs are caught in checked-mode.

[asgerf, 2015-09-29 11:09:06]

Committed patchset #6 (id:120001) manually as
808f5e2228df9c41a909fcc31c481334a14a1d2b (presubmit successful).