Dart: Makes timing of sending trace events configurable.

Dart: Makes timing of sending trace events configurable.

There was a change to send trace events only when tracing stops.
This is appropriate for some situations but not others.
This change adds an option to configure the timing of sending trace
events.

BUG=
R=johnmccutchan@google.com

Committed: https://chromium.googlesource.com/external/mojo/+/4d9fcdb7903dad4e2e5537c6cea147c52f17aa11

[zra, 2015-10-14 17:23:53]

zra@google.com changed reviewers:
+ johnmccutchan@google.com

[qsr, 2015-10-14 17:31:09]

What is the issue with always getting the trace at the end? Thta's what the
tracing does in C++?

[zra, 2015-10-14 17:39:04]

On 2015/10/14 17:31:09, qsr wrote:
> What is the issue with always getting the trace at the end? Thta's what the
> tracing does in C++?

It makes writing microbenchmarks for Dart difficult since one would also have to
consider any GC effects when accumulating a possibly very large and long-lived
list of strings. Likely, the Dart VM handles this situation gracefully, but I
would rather have it factored out entirely so that we can have high confidence
that we are measuring what we think we're measuring. This is more relevant for
the round trip time benchmarks than for the start-up benchmarks.

[Cutch, 2015-10-15 16:44:25]

lgtm

[zra, 2015-10-15 16:58:25]

Committed patchset #1 (id:1) manually as
4d9fcdb7903dad4e2e5537c6cea147c52f17aa11 (presubmit successful).