source maps broken for class declarations

[jmesserly]

using http://evanw.github.io/source-map-visualization/ for sunflower ... we're pretty good at expressions and identifiers and function bodies. But hovering on the classes/constructors/methods lines look really bogus. Granted, those aren't exactly "executable" in Dart, but still, we should do better.

Field initializers also looked broken.

[jmesserly]

Method cascades are broken too. e.g. ..closePath() instead of mapping to closePath it seems to map to exports.context