• Use BsonUndefined instead of null
• java.lang is always imported, so don't need to use the full name.
• Use SimpleDateFormat instead of toString()
• Use toHexString instead of toString for ObjectId
• Use BINARY instead of OLD_BINARY for Binary subtype.
• Use Document constructor, so generate "{x: 1}" as "new Document("x", 1)" and not "new Document().append("x", 1)"
• Remove Code.toString() for JS and Shell because the result is just "[object Object]"

https://github.com/mongodb-js/bson-compilers/pull/82 

https://github.com/mongodb-js/bson-compilers/pull/84

Comments from: https://github.com/mongodb-js/bson-compilers/pull/76