Use blocking file access in tools/test.dart for multitest creation.

Use blocking file access in tools/test.dart for multitest creation.

Speeds up test.dart by a factor of 2, and test.dart --list by a factor of 6.
Test.dart is now only 15% slower than test.py.

BUG=
TEST=tools/test.dart

Committed: https://code.google.com/p/dart/source/detail?r=2889

[Bill Hesse, 2011-12-30 16:12:23]

Speeds up multitest creation by a factor of 10.  This was the main performance
problem in test.dart.  It is now within 15% of test.py's performance.

Speeds up test.dart --list from 2:30 to 0:25, and test.dart from 4:30 to 2:20 
(test.py) is at 2:00.  These times are to get to 99% - there are a few tests
timing out at the end.

http://codereview.chromium.org/9003017/diff/1/tools/testing/dart/multitest.dart
File tools/testing/dart/multitest.dart (left):

http://codereview.chromium.org/9003017/diff/1/tools/testing/dart/multitest.da...
tools/testing/dart/multitest.dart:115: 
Oops.

[Søren Gjesse, 2012-01-02 10:03:50]

LGTM

Maybe we should open a bug on changing this back when the async operations have
been optimized.