Use Window.performance.now if available, with correct frequency value

Use Window.performance.now if available, with correct frequency value


BUG=

Committed: https://code.google.com/p/dart/source/detail?r=15353

[Emily Fortuna, 2012-11-26 23:31:30]

This is the CL that will never end!
So my last version didn't work because I misunderstood that
window.performance.webkitNow() actually still returns a value in milliseconds,
even though it is more accurate to the microsecond. 

So, I've modified one line (the one with the comment below)

https://codereview.chromium.org/11316181/diff/2001/sdk/lib/_internal/compiler...
File sdk/lib/_internal/compiler/implementation/lib/js_helper.dart (right):

https://codereview.chromium.org/11316181/diff/2001/sdk/lib/_internal/compiler...
sdk/lib/_internal/compiler/implementation/lib/js_helper.dart:543: return (1000 *
JS('num', '#.webkitNow()', performance)).floor();
This is the line that's different. The key thing I misunderstood previously is
even though window.performance.webkitNow() returns up to microsecond precision,
it still returns it in millisecond granularity. So, since now() is supposed to
only return an int, I multiply by 1000, and then take the floor.

[blois, 2012-11-26 23:33:02]

lgtm

[ahe, 2012-11-27 14:37:33]

LGTM