add sourcesContent to source-maps

    traceur adds runtime sources dynamically with file names ending in @runtime.
    These sources do not exist as network resources or files, so devtools gets
    confused and issues a bunch of errors with xxx@runtime URLs which confuses
    developers.

    By including the source in the source-map the messages disappear and
    we can see the runtime source if we single step. Unless we use workspaced
    mappings, see crbug 312331