Added preemption option to d8

Added -p option to d8 that runs a list of source files in a separate
thread with preemption enabled.

[Erik Corry, 2009-01-20 13:28:30]

LGTM.  I think one single Locker object can solve all 3 locking issues.

http://codereview.chromium.org/18404/diff/1/2
File src/d8.cc (right):

http://codereview.chromium.org/18404/diff/1/2#newcode449
Line 449: while (ptr) {
Test against NULL should be explicit.

http://codereview.chromium.org/18404/diff/1/2#newcode453
Line 453: if (next) {
Test against NULL should be explicit.

http://codereview.chromium.org/18404/diff/1/2#newcode457
Line 457: ptr = next;
ptr = NULL would be clearer.

http://codereview.chromium.org/18404/diff/1/2#newcode475
Line 475: Context::Scope context_scope(evaluation_context_);
This should be protected by a locker.  Also the place where it goes out of scope
- is there a nontrivial destructor?

http://codereview.chromium.org/18404/diff/1/2#newcode496
Line 496: Locker::StartPreemption(10);
You only need to do this once, not once per file.

http://codereview.chromium.org/18404/diff/1/2#newcode505
Line 505: // Use all other arguments as names of files to load and run.
If the user uses both -p files and straight js files on the command line we need
a lock here.

http://codereview.chromium.org/18404/diff/1/2#newcode520
Line 520: RunShell();
and here.

[Christian Plesner Hansen, 2009-01-20 14:13:45]

http://codereview.chromium.org/18404/diff/1/2
File src/d8.cc (right):

http://codereview.chromium.org/18404/diff/1/2#newcode449
Line 449: while (ptr) {
Fixed (times 3).

http://codereview.chromium.org/18404/diff/1/2#newcode475
Line 475: Context::Scope context_scope(evaluation_context_);
I don't think so -- there can only be one thread during initialization.

http://codereview.chromium.org/18404/diff/1/2#newcode496
Line 496: Locker::StartPreemption(10);
Right, but I don't want to enable it without the -p option, and this way I don't
have to keep track of when I first meet a -p.

http://codereview.chromium.org/18404/diff/1/2#newcode505
Line 505: // Use all other arguments as names of files to load and run.
Fixed

http://codereview.chromium.org/18404/diff/1/2#newcode520
Line 520: RunShell();
Fixed