chore(reporter): record lexer benchmark results

This initial version sends benchmark metrics to the backend server.

Visit https://ng-dash.appspot.com/commit/fb2aa60a584dd4b2bbb3d30e8fcd4c588f8669ed
or https://ng-dash.appspot.com/all to see an example.  (github:
https://github.com/chirayuk/ng-dash)

In this version, only Dart code run from the command line is able to
record such reports.

The code in benchmark/_reporter.dart handles sending the data to the
server.  A future version will mode this code into a locally running
proxy server enabling browser based Dart code to send such reports while
still including the information from the build environment.

Reading data from the report server does not require any auth.  See a
simple example at https://github.com/chirayuk/ng-dash/commit/ex_01

Writing data requires authentication.  Specifically, the REST server
requires two cookies set:  user_email and user_secret.  (user_email does
not have to be a valid e-mail just like AppEngine user e-mails.)  Of
these, user_secret is meant to be secret and should be safeguarded.  For
Travis, user_email is "travis-ci.org" and user_secret is encrypted in
the .travis.yml.  Refer
http://docs.travis-ci.com/user/build-configuration/#Secure-environment-variables
for details on how this is done.
you can drop String here