Use a consistent style for named vs. default exports

[anandthakker]

My proposal:

• No exported "namespace objects" (Replace exported objects with named exports #6335) -- modules should export either classes or functions, with an occasional exception as needed for stubbing.
• If a module exports one thing, use a default export
• If a module exports multiple things, use named exports only

Whatever we decide, it should be backed up by lint rules.

[jfirebaugh]

Counterproposal:

• If a module exports something with the same name as the file name (modulo case), it should be the default export.
• Anything else should be a named export.

[anandthakker]

When you put it that way, I'm sold