Feature-detect browser for assignment to __proto__.

Feature-detect browser for assignment to __proto__.

Assigning to __proto__ is a fast way to stitch together a number of
object literals into a prototype chain without copying all the
properties.

Unfortunately this leads to very slow execution on Firefox (and
jsshell).  So now we feature-detect Chrome (and d8) since that is the
only browser we know currently can use the 'fast' startup path without
penalizing subsequent performance.

This speeds up some jsshell benchmarks by 30%.

R=het@google.com

Committed: https://github.com/dart-lang/sdk/commit/5923966da3388666f5d7692f2876666cad8b9255

[sra1, 2016-07-22 20:19:24]

sra@google.com changed reviewers:
+ het@google.com

[Harry Terkelsen, 2016-07-22 20:21:49]

lgtm

[sra1, 2016-07-23 18:42:41]

Description was changed from

==========
Feature-detect browser for assignment to __proto__.

Assigning to __proto__ is a fast way to stitch together a number of
object literals into a prototype chain without copying all the
properties.

Unfortunately this leads to very slow execution on Firefox (and
jsshell).  So now we feature-detect Chrome (and d8) since that is the
only browser we know currently can use the 'fast' startup path without
penalizing subsequent performance.

This speeds up some jsshell benchmarks by 30%.
==========

to

==========
Feature-detect browser for assignment to __proto__.

Assigning to __proto__ is a fast way to stitch together a number of
object literals into a prototype chain without copying all the
properties.

Unfortunately this leads to very slow execution on Firefox (and
jsshell).  So now we feature-detect Chrome (and d8) since that is the
only browser we know currently can use the 'fast' startup path without
penalizing subsequent performance.

This speeds up some jsshell benchmarks by 30%.

R=het@google.com

Committed:
https://github.com/dart-lang/sdk/commit/5923966da3388666f5d7692f2876666cad8b9255
==========

[sra1, 2016-07-23 18:42:43]

Committed patchset #2 (id:20001) manually as
5923966da3388666f5d7692f2876666cad8b9255 (presubmit successful).