Make safe points in generated code invisible to the debugger

Make safe points in generated code invisible to the debugger

- remove some PC descriptors that are part of implicit, synthesized
  code, e.g. generated null checks in == methods, implicit constructors,
  implicitly generated code in user constructors, etc.
- ignore single step callbacks on PC descriptors that are part of
  synthesized code.

R=regis@google.com

Committed: https://code.google.com/p/dart/source/detail?r=31966

[regis, 2014-01-21 20:11:40]

LGTM

https://codereview.chromium.org/137003006/diff/100001/runtime/vm/parser.cc
File runtime/vm/parser.cc (right):

https://codereview.chromium.org/137003006/diff/100001/runtime/vm/parser.cc#ne...
runtime/vm/parser.cc:1184: new ReturnNode(Scanner::kDummyTokenIndex,
load_field);
A comment indicating why it is important to pass a dummy token pos would be
useful for code maintenance. Or maybe an alias of Scanner::kDummyTokenIndex with
a meaningful name would work too.

https://codereview.chromium.org/137003006/diff/100001/runtime/vm/parser.cc#ne...
runtime/vm/parser.cc:1228: current_block_->statements->Add(new
ReturnNode(Scanner::kDummyTokenIndex));
ditto

https://codereview.chromium.org/137003006/diff/100001/runtime/vm/parser.cc#ne...
runtime/vm/parser.cc:1238: const intptr_t invisible_pos =
Scanner::kDummyTokenIndex;
Would be unnecessary if you declare an alias of Scanner::kDummyTokenIndex as
suggested above.

https://codereview.chromium.org/137003006/diff/100001/runtime/vm/parser.cc#ne...
runtime/vm/parser.cc:2424: const intptr_t dummy_pos = Scanner::kDummyTokenIndex;
ditto

[hausner, 2014-01-21 21:49:50]

Committed patchset #2 manually as r31966 (presubmit successful).

[hausner, 2014-01-21 21:50:52]

Thank you.

As discussed offline, I'll introduce an alias for kDummyTokenIndex in a separate
change since it will affect many files.