dart2js cps: Improve rewriting of == to identical.

dart2js cps: Improve rewriting of == to identical.

We now rewrite `x == y` if x is known to have a == implementation that
corresponds to identical(). Previously it would only do this when x was
num, string, or bool.

BUG=
R=karlklose@google.com

Committed: https://github.com/dart-lang/sdk/commit/703e968620ff990127a77bcfd25e2846573c82c4

[asgerf, 2015-08-12 09:34:06]

asgerf@google.com changed reviewers:
+ karlklose@google.com

[asgerf, 2015-08-12 09:34:06]

https://codereview.chromium.org/1286993003/diff/1/pkg/compiler/lib/src/cps_ir...
File pkg/compiler/lib/src/cps_ir/type_propagation.dart (right):

https://codereview.chromium.org/1286993003/diff/1/pkg/compiler/lib/src/cps_ir...
pkg/compiler/lib/src/cps_ir/type_propagation.dart:890: if (left.isNullConstant
|| right.isNullConstant) {
The check on left is technically redundant, since the analysis below also
handles it. It just seems more uniform to have it.

[karlklose, 2015-08-12 10:00:28]

LGTM!

[asgerf, 2015-08-12 11:13:51]

Committed patchset #1 (id:1) manually as
703e968620ff990127a77bcfd25e2846573c82c4 (presubmit successful).