Implement constructor tearoffs support in web debuggers

[eernstg]

This issue tracks support of the constructor tearoffs feature. See the enclosing meta-issue for details.

Update

Played with that a bit with @jakemac53's help.

Discovered some setup issues (not urgent as I can work around them by building SDK with tearoffs enabled)

• --enable-experiment flag is not processed correctly in build_runner:
  • fixed in fix experiment support in ddc build#3178
• --enable-experiment flag is not processed correctly in build_daemon:
  • issue: build_daemon does not support --enable-experiment option. build#3180
• --enable-experiment flag is not passed to the expression compiler in webdev:
  • [dartdevc] Add --enable-experiment options to metadata #47068

Issues discovered during debugging apps with constructor tearoffs:

• Internal stack frames are visible when debugging code that uses constructor tearoffs webdev#1399

[sigmundch]

@annagrin - feel free to move this to another repo if it's more appropriate to have it there (e.g. webdev)

[annagrin]

@sigmundch I will keep it here for now, if any work will be needed it would probably include SDK as well as webdev.

[devoncarew]

@annagrin - do you know if there will be work to do here? We expect ctor tearoffs to be feature complete early October. Note that there is another issue for any vm service protocol changes (#46240); I'm not sure if there are any but @bkonyi would know.

[annagrin]

@devoncarew I run the debugger with code using constructor tearoffs and it works as expected.

Work remaining:

• Expression evaluation tests:
  Add expression evaluation tests for constructor tearoffs #47214
  @nshahan would you be interested in taking over this task?

• Extra stack frame:
  Internal stack frames are visible when debugging code that uses constructor tearoffs webdev#1399
  We cannot easily solve this without debug symbols work:
  https://github.com/orgs/dart-lang/projects/29#card-68041615
  that will take longer than the constructor tearoff feature deadline.
  We could try using some naming heuristics to work around it if needed, but
  I'd suggest to wait and collect user reaction first.

So as long as the expression evaluation tests are done we should be fine with shipping.

[nshahan]

Writing some expression eval tests is on my todo list. I assigned that to myself.

[devoncarew]

@nshahan - checking on on this. Are the tests all that remain of this issue? Is that tracked somewhere else?

[nshahan]

@devoncarew I landed the expression evaluation tests already (that issue is now closed) if that is what you are asking about.

@annagrin At this point do we need to implement the changes related to experiments for the constructor tearoffs feature? Is there anything else from the debugger side that we need to do for the feature?

[devoncarew]

Ah, thanks - I missed the issue above.

[sigmundch]

Quick update: all blocking work is complete at this time

We still need to plumb flags so that in the future it is easier to test features before enabling them, but that's unrelated to this issue.

As mentioned above, we'd like to eventually fix dart-lang/webdev#1399 (don't show synthetic frames). This is not a release blocker, but will be part of the project to enable symbol metadata in the debugger (needed also for object inspection support)