dynamic values and generic invocations

[CeylonMigrationBot]

[@gavinking] The discussion in #3710 raises the issue of the semantics of a generic invocation with an untyped argument.

We need to validate that the js compiler does the right thing here. Not 100% certain what the "right thing" is, but I think:

• the typechecker should reject code where the inferred type args involve an unknown type, and
• the backend must validate args against the inferred type args.

Whatever, we need to make sure we have proper tests.

[Migrated from ceylon/ceylon-js#215]

[CeylonMigrationBot]

[@chochos] it's hard to write proper tests for something we don't even know how it's supposed to work...