[dev_compiler] Allow compiling `this` in extension methods during expression compilation for web

[annagrin]

Update: VM support has landed, reuse this issue to extend the support to compilation to javascript:

48d8225

VM:

dwds:

Currently both VM and dwds cannot evaluate this in extension methods.
Not sure about VM, but on the web the problem is a scope mismatch:

• the scope in kernel contains #this, not this
• the expression compiled is this, so compilation fails
• compiling #this also fails because the incremental compiler does not allow # in variables.

Not sure if the solution lies in CFE (changing the name?) or the users of it (adding this to scope?) or both.

[annagrin]

/cc @johnniwinther @jensjoha I appreciate your suggestions here.

[johnniwinther]

We need to pipe new information to the BodyBuilder in the CFE since it needs to know that it should resolve implicit and explicit this into a VariableGet of a special parameter.

What information do you have available at the call site? Do you know that you are in a extension member? Do you know if the member is a static extension member? Do you know which variable serves as the synthesized #this? Do you know which extension you are in?

[annagrin]

What information do you have available at the call site? Do you know that you are in an extension member?

@johnniwinther For web scenario, the expression compiler has access to full kernel - for example, it traverses the kernel to find all variables in scope to pass to the incremental compiler.

Expression compiler could leverage mapping information potentially stored in kernel and map the variables from dart to kernel before passing to the incremental compiler, or just give it enough information if needed. But I think it has the same information as incremental compiler in this case (plus the line number), so maybe the incremental compiler can do the mapping as well?

Do you know which variable serves as the synthesized #this?

I was naively sure it is always this for non-static extensions:) So I am not sure about that - could you please give some examples? Expression compiler has the variable nodes for all variables in scope, but currently just passes names to the incremental compiler.

I am not familiar with the VM implementation though.

[jensjoha]

VM support has landed. Is this enough to get you unstuck @annagrin ?

[annagrin]

@jensjoha thanks, this is very helpful! I will keep this issue for extending your work to expression compilation to javascript, will let you know if I run into any problems.