Tests for trailing commas in function arg lists (#733)

...xpressions/arrow-function/params-trailing-comma-length.js

...guage/expressions/arrow-function/params-trailing-comma.js

test/language/expressions/call/trailing-comma.js

...e/expressions/function/params-trailing-comma-arguments.js

...uage/expressions/function/params-trailing-comma-length.js

test/language/expressions/function/params-trailing-comma.js

...expressions/generators/params-trailing-comma-arguments.js

...ge/expressions/generators/params-trailing-comma-length.js

.../language/expressions/generators/params-trailing-comma.js

...ject/method-definition/params-trailing-comma-arguments.js

.../object/method-definition/params-trailing-comma-length.js

...ns/object/method-definition/params-trailing-comma-rest.js

...essions/object/method-definition/params-trailing-comma.js

test/language/rest-parameters/params-trailing-comma-rest.js

...ments/class/definition/params-trailing-comma-arguments.js

...atements/class/definition/params-trailing-comma-length.js

...statements/class/definition/params-trailing-comma-rest.js

...uage/statements/class/definition/params-trailing-comma.js

...ge/statements/function/params-trailing-comma-arguments.js

...guage/statements/function/params-trailing-comma-length.js