Update language spec to v0.6 (M6)

Staged at:
https://spec-dot-dart-lang.appspot.com/docs/spec/
https://spec-dot-dart-lang.appspot.com/docs/spec/latest/dart-language-specification.html
https://spec-dot-dart-lang.appspot.com/docs/spec/latest/dart-language-specification.pdf

Review URL: https://chromiumcodereview.appspot.com/23674006