Support "--checked" in the dart2js Compiler API

[nex3]

Currently, when invoking dart2js programmatically via the Compiler API, all of the options must be passed by string (issue #17754) and there's no validation of these options (issue #16031), nor any documentation. To make matters worse, it doesn't accept the same options as the command-line interface. In particular, the command-line interface allows "-c" or "--checked" to enable checked mode, but the Compiler interface only supports "--enable-checked-mode".

[floitschG]

cc @johnniwinther.