Flutter Widget Inspector doesn't work with new kernel const format

[vsmenon]

The flutter widget inspector transform expects to be able to access the original const constructors. It can't do that with new consts.

Per @mraleph : Flutter 3xHEAD started failing exactly at the enabling commit. [https://ci.chromium.org/p/dart/builders/ci.sandbox/flutter-engine-linux/2068]

@jacob314 is investigating if there is a way to proceed.

[askeksa-google]

We discussed this on the front-end team, and here's our proposal for fixing it:

• Add a flag to the front-end CompilerOptions which will make the constant evaluator leave constructor invocations unevaluated (as constructor invocations, rather than the InstanceCreation nodes they are usually translated into when unevaluated).
• Enable the flag when compiling code that is to be transformed by the Wdiget transformer.
• Instantiate a constant evaluator in the Widget transformer.
• Make the Widget transformer recurse into unevaluated constants to find (and transform) the constructor invocations inside. When it comes back out, it will call its constant evaluator to evaluate the constant. It needs to do so for all constants (not just the unevaluated ones) to do correct canonicalization of all constants.
• Also explicitly evaluate all new constants created by the Widget transformer (if they are not already inside a constant).

@jacob314 what do you think of this plan? It means the front-end run will need to know whether this transformation is happening later, so those two steps can't run independently. Does this fit how the compilation process in the Widget Inspector works already?

[jacob314]

Can you describe the benefits of this plan relative to running the Widget Inspector transform before any const evaluation?

What do you expect the performance impact of this proposal will be? The kernel transformer tracking widget creation locations needs to be fast as it is run as part of all debugging runs of flutter applications initiated by IDEs.

If I'm understanding it correctly consts like

const [1,2,3]
const {'a': 3, 'b': 5}

would still be evaluated before the widget transformer
but consts such as

const Text('foo')

and

const [Text('foo'), Text('bar)]

would not be evaluated. Is that correct?