Don’t set more than one code breakpoint per token position

Don’t set more than one code breakpoint per token position

Some time ago when operator== calls had inlined null checks at
the call site, the debugger had to instrument multiple code breakpoints
that were all mapping to the same source position. This is no longer
necessary. This change makes sure we set a code breakpoint only at
the lowest compiled code address, so that expressions like i++ no longer
result in 2 or 3 breakpoints (one each for load filed, add, store field).

Fixes 18397.

R=srdjan@google.com

Committed: https://code.google.com/p/dart/source/detail?r=35510

[srdjan, 2014-04-28 23:40:15]

lgtm

[hausner, 2014-04-28 23:40:43]

Committed patchset #2 manually as r35510 (presubmit successful).