Introduce is_debuggable state bit on function objects

Introduce is_debuggable state bit on function objects

Simplify debugger logic. Any function can be marked as
non-debuggable when is is created. The debugger no longer
needs a heuristic which functions are debuggable.
Mostly used for synthetic, generated functions that
have no source code, e.g. async code, implicit getters and
setters, implicit constructors, forwarding constructors,
dispatcher functions.

Committed: https://code.google.com/p/dart/source/detail?r=42570

[hausner, 2014-12-29 20:25:39]

hausner@google.com changed reviewers:
+ asiva@google.com

[siva, 2014-12-29 22:11:58]

The CL in general looks good, I have two questions
- native functions should also be marked as not debuggable?
- we have another flag 'is_visible' which stops a function
  from being visible on the stack trace (this flag is also
  overloaded to mean reflective access should be
  prevented).
  If a function should not be visible on the stack trace
  shouldn't it also be set as not debuggable.
  Also should functions that have been marked as not
  debuggable be hidden in the stacktrace?

[hausner, 2014-12-29 22:26:59]

Good point about native functions. I changed the parser to mark them as
non-debuggable.

Visibility in the stack trace and debugging are somewhat independent features.
There is some overlap, but we are hiding some internal functions that we might
want to be able to debug.

[hausner, 2014-12-29 22:31:24]

Committed patchset #3 (id:40001) manually as r42570 (presubmit successful).