Add debugger steps in local variable assignments

Add debugger steps in local variable assignments

This allows the debugger to single step and break on local
variable assignments in unoptimized code. Checks are only added if
the right hand side of the assignment has no safepoints where
the debugger would stop anyway.

Interestingly, generated code size is only 0.1% bigger, so the
cost is negligible.

Addresses issue 10403.

R=iposva@google.com

Committed: https://code.google.com/p/dart/source/detail?r=31575

[hausner, 2014-01-06 23:59:17]

Bonus fix: eliminate debug step checks in native function returns.

[Jacob, 2014-01-07 00:02:22]

This is great! Are there any other remaining cases where a valid Dart statement
does not have any safe points?

[hausner, 2014-01-07 00:05:18]

continue and break come to mind. Local variable declarations that have no
explicit initializer.

[Florian Schneider, 2014-01-07 10:01:42]

https://codereview.chromium.org/125033007/diff/60001/runtime/vm/flow_graph_bu...
File runtime/vm/flow_graph_builder.cc (right):

https://codereview.chromium.org/125033007/diff/60001/runtime/vm/flow_graph_bu...
runtime/vm/flow_graph_builder.cc:3216: if (node->value()->IsLiteralNode()) {
This only handles literals like "v = 123". Where are local variable loads on the
rhs handled?

[hausner, 2014-01-07 17:00:30]

https://codereview.chromium.org/125033007/diff/60001/runtime/vm/flow_graph_bu...
File runtime/vm/flow_graph_builder.cc (right):

https://codereview.chromium.org/125033007/diff/60001/runtime/vm/flow_graph_bu...
runtime/vm/flow_graph_builder.cc:3216: if (node->value()->IsLiteralNode()) {
On 2014/01/07 10:01:42, Florian Schneider wrote:
> This only handles literals like "v = 123". Where are local variable loads on
the
> rhs handled?

Nice catch, thank you. Added local variable load nodes, since they don't produce
a safe point.

[Ivan Posva, 2014-01-07 20:59:31]

LGTM -ip

[hausner, 2014-01-07 21:10:52]

Committed patchset #3 manually as r31575 (presubmit successful).