Write barrier verification: Accept zapped scavenger memory to fix issue 21851.

This showed up under --observe because of semi-space reuse between isolates.
Unclear how to write reliable regression test for this.

BUG=dart:21851
R=iposva@google.com

Review URL: https://codereview.chromium.org//797283003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42345 260f80e4-7a28-3924-810f-c04153c831b5
Pass condition result explicitly. Do not imply the existence of a status
register. MIPS does not have one.
This is in preparation of another cl improving generated code for conditional
expressions and branches on MIPS.

R=srdjan@google.com

Review URL: https://codereview.chromium.org//798643003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42346 260f80e4-7a28-3924-810f-c04153c831b5
Avoid NPE in package resolver if there's a problem running 'pub list'

R=paulberry@google.com

Review URL: https://codereview.chromium.org//799063002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42347 260f80e4-7a28-3924-810f-c04153c831b5
Deoptimize the world when single stepping.

It is possible that we may be stepping into an optimized frame
otherwise.  In fact, the current frame may be optimized if the isolate
was interrupted instead of hitting a breakpoint.

BUG=
R=hausner@google.com

Review URL: https://codereview.chromium.org//797343002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42348 260f80e4-7a28-3924-810f-c04153c831b5
Make instrumentation available in engine

R=scheglov@google.com

Review URL: https://codereview.chromium.org//798123002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42349 260f80e4-7a28-3924-810f-c04153c831b5