The root cause for the bug is that the positions assigned to desugared
code was inconsistent with the source ranges of block scopes.
Since the fact that the position is assigned causes the debugger to
break at the parser-generated statement, the fix is to remove positions
from those nodes that we do not want to break on.

The CL also teaches Hydrogen to tolerate these cases.

R=adamk@chromium.org,rossberg@chromium.org
BUG=chromium:468661
LOG=Y

Review URL: https://codereview.chromium.org/1032653002

Cr-Commit-Position: refs/heads/master@{#27424}