[build] Specify outptu JS language version (#443)
[build] Specify outptu JS language version (#443)

Explicitly tell the Closure Compiler to make the resulting files to
conform to the JavaScript specification not newer than year 2015,
because our app's minimum_chrome_version is still 48 (released in
2015/2016).

This is an additional safety measure: most likely we aren't using
unsupported constructs anyway, but the explicit flags will make sure of
that.
googlechromelabs/chromeos_smart_card_connector,googlechromelabs/chromeos_smart_card_connector,GoogleChromeLabs/chromeos_smart_card_connector,GoogleChromeLabs/chromeos_smart_card_connector,GoogleChromeLabs/chromeos_smart_card_connector,googlechromelabs/chromeos_smart_card_connector,googlechromelabs/chromeos_smart_card_connector,GoogleChromeLabs/chromeos_smart_card_connector,GoogleChromeLabs/chromeos_smart_card_connector,googlechromelabs/chromeos_smart_card_connector,googlechromelabs/chromeos_smart_card_connector,GoogleChromeLabs/chromeos_smart_card_connector