Introduce --no-prune-dead-locals flag to disable dead locals pruning.

Disabling this optimization regresses code quality but improves debugging experience.

R=fschneider@google.com
BUG=

Review URL: https://codereview.chromium.org//690043002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41423 260f80e4-7a28-3924-810f-c04153c831b5