JS compiler generates code that clobbers existing namespaces

The JavaScript compiler currently generates code that will clobber an already-existing namespace. It should check if the namespace exists before initializing it to {}. Example code pre-patch, assuming the .thrift file has the declaration namespace js CompanyName.ModuleName:

var CompanyName = {};
CompanyName.ModuleName = {}

Becomes this after patch:

if (typeof CompanyName === 'undefined') CompanyName = {};
if (typeof CompanyName.ModuleName === 'undefined') CompanyName.ModuleName = {};