Fix a small oversight in ES6 conversion in which we tried to use a STRING_KEY node as a name node.

It turns that this was dead code because visitStringKey had already been called.
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=75225904
Transpile array patterns in param lists.
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=75231533