Use 'assert' module in generateExterns.
Use 'assert' module in generateExterns.

The 'assert' module will throw an error that will crash the script.
This ensures an error is caught instead of causing silent problems.

Change-Id: I47d54924bff3a83b9897fd84a85142e237f18da6

tvoli/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,tvoli/shaka-player