Use a Map instead of an Obejct to store tests
Use a Map instead of an Obejct to store tests

This change has two reasons:
- The object was actually used as a map
- Using an object leads some problems with the
  private_class_fields/constructor.js test, since
  `tests[test_name] || {}` returned the Obejct
  constructor instead of an empty object.

hzoo/babel,hulkish/babel,shuhei/babel,guybedford/babel,babel/babylon,samwgoldman/babel,chicoxyzzy/babel,maurobringolf/babel,jridgewell/babel,kellyselden/babel,claudiopro/babel,kaicataldo/babel,claudiopro/babel,claudiopro/babel,babel/babel,Skillupco/babel,hzoo/babel,hulkish/babel,kaicataldo/babel,maurobringolf/babel,jridgewell/babel,jridgewell/babel,existentialism/babel,cbishopvelti/babylon,guybedford/babel,chicoxyzzy/babel,PolymerLabs/babel,hzoo/babel,kellyselden/babel,hulkish/babel,kellyselden/babel,kaicataldo/babel,Skillupco/babel,guybedford/babel,kellyselden/babel,kaicataldo/babel,shuhei/babel,babel/babel,vadzim/babel,chicoxyzzy/babel,Skillupco/babel,vadzim/babel,samwgoldman/babel,maurobringolf/babel,jridgewell/babel,chicoxyzzy/babel,samwgoldman/babel,existentialism/babel,PolymerLabs/babel,babel/babel,shuhei/babel,existentialism/babel,hulkish/babel,babel/babel,Skillupco/babel,PolymerLabs/babel,hzoo/babel