[Patterns] Web debugging support

[itsjustkevin]

[annagrin]

Currently debugging experience for pattens is not very clean:

dart-lang/webdev#1974

[itsjustkevin]

As dart-lang/webdev#1974 is close, is there anything else we need to call this done @annagrin?

[vsmenon]

@elliette - anything you're planning to land here?

[elliette]

We should be all good here:

• Initial pattern support was implemented in Prepare for variable names changes due to patterns support in DDC webdev#2042, included in DWDS 18.0.2
• Follow-up polish was implemented in Don't show non-instantiated variables webdev#2061, included in DWDS 19.0.0

DWDS 19.0.0 has been rolled into Flutter: flutter/flutter#124138