dynamic dispatch doesn't handle extensions

[jmesserly]

It won't find methods like: list[core.$where]

note, if we fix related issue: #138, then we only need to look for extensions when we get a native Array type. If we get a real Dart subtype, it will have the named method (e.g. where) ... we have similar issues right now with dynamic sends to all String/int/num/double/bool members.

[jmesserly]

@jacob314 @jmesserly Do you guys have thoughts on how to handle this?

yeah, what I would try, we have registerExtension which informs dart_runtime about the Array<->List correspondence, that should be enough to find the symbols and ensure we add them to a dynamic dispatch table. Then everywhere we currently do obj[name] becomes a slightly more sophisticated check, that also knows to check for symbol too.

For certain names, we probably can just check the symbol first. Although note the related issues in #138.