WebPageTranscoder cannot pass name as argument.

(Caught by recent destructuring check.)

Review URL: https://github.com/google/traceur-compiler/pull/945

Closes #945.
Report file not found as 'File not found'

Review URL: https://github.com/google/traceur-compiler/pull/946

Closes #946.
Upgrade CodeMirror to v4

This allows multiple selections which are awesome.

Enable the sublime keymap. Based on demo at
http://codemirror.net/demo/sublime.html