[Wildcard Variables] Dart2JS Implementation about sdk

[sigmundch]

Thanks @kallentu, similar comments to what I posted in the DDC issue:

• the work here depends a lot on what kind of lowering we end up doing in the CFE
• the lowering will likely convert the wildcard variables into something, potentially a variable declaration that is never read.
• if the lowering uses the existing representation of variable declarations and guarantees uniqueness of variable names like today parameters have, then the backend may not need to do anything special for it to generate valid JavaScript. However, if the variable is represented by a new kernel node or a node that breaks current invariants, we may need some relatively small amount of work to emit valid JavaScript function declarations. The work could be similar to what DDC already does for temporary variables in Let expressions.