Support use of external strings as inputs to LoadCodeUnitsInstr.

R=vegorov@google.com
BUG=

Review URL: https://codereview.chromium.org//765743003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42099 260f80e4-7a28-3924-810f-c04153c831b5
Initialize offsets also for the unreachable branch (clang -Wsometimes-uninitialized).

TBR=vegorov@google.com
BUG=

Review URL: https://codereview.chromium.org//779903003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42100 260f80e4-7a28-3924-810f-c04153c831b5
Disable RegExp tests when using JSCRE backend.

TBR=vegorov@google.com
BUG=

Review URL: https://codereview.chromium.org//761663006

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42101 260f80e4-7a28-3924-810f-c04153c831b5