Fix reading from stdin

[lahmatiy]

compile takes a list of sources in format { src: "js" }. When read from stdin its content doesn't wrap into object and compile returns an empty output.

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[lahmatiy]

I signed it!

[googlebot]

CLAs look good, thanks!

[lahmatiy]

Oh, I found that another PR (https://github.com/google/closure-compiler-js/pull/56) does the same.