Make let generation the default.

[jdalton]

Now that Node 4 is the min bar let by default would be rad. For the case of the REPL I'm guessing it could be configured false for REPL use. I'm missing the option toggle for the REPL use.

[jdalton]

While working on this PR I noticed some nested import tests failing

switch (i) {
case 0:
  import { a as x } from "./abc";
  break;
case 1:
  import { b as x } from "./abc";
  break;
}

because:

SyntaxError: Identifier 'x' has already been declared

which makes sense because it's let. This is something to keep in mind as a toggle for nested import statements.

[jdalton]

Being able to set options for the REPL is probably a good thing despite the current nested import let issue.

[benjamn]

Re: switch case scope collisions, I'm tempted to wrap the case: bodies with braces, though I guess that would be an observable semantic change. Where should the let declarations go, if we wanted to make your example work?

[jdalton]

Where should the let declarations go, if we wanted to make your example work?

If nested imports was configurable then in the case of it being disabled let could be used.
As it is, let shouldn't really be used while nested imports is enabled since enabling it can cause errors.

I'll hold off on this PR until a few other things are in place.