Fix uses of fuzzy arrows in dart2js and re-enable hint

[leafpetersen]

The treatment of dynamic as bottom when it occurs in the argument position of a function type is being deprecated and removed in Dart 2.0. I am adding a hint to warn on uses of this, and will land a CL disabling this hint in the dart2js analysis_options.yaml.

The line uses_dynamic_as_bottom: ignore should be removed from /pkg/compiler/analysis_options.yaml once the code as been fixed up appropriately.