fix js-interop tearoffs and optional arguments

[sigmundch]

The current behavior in the new js-interop is to omit default arguments that were not specified when making an interop call. This is so that the JS side can fill in default values. The SSA implementation does this even when default values are specified. This doesn't work however when using a tearoff of the function: in that case we do include the default arguments (in dart2js).

It might be possible to fix tearoffs, but the behavior might still be confusing. @jacob314 - what do you think about changing things as follows:

• if no default value is specified, omit the argument (like today)
• if a default value is specified, include it in the call (making direct calls and tearoffs consistent).
• add a hint/warning that you most likely don't want to specify these default values in the first place

[jacob314]

We need to make this an error.

[sigmundch]

sounds good to make it an error - in Dartium, we likely want to pass the arguments that have a default value so that the default behavior is consistent (if users choose to ignore the error message).

Should we file a separate bug for dartium on this?

[jacob314]

Yep added #25768