export default broken for Class expression with target ES6 and commonjs module

[ghalle]

Using typescript@next and the following compiler options:

"compilerOptions": {
  "target": "ES6",
  "module": "commonjs",
}

A class expression being default exported will be incorrectly compiled into a named export instead.

Example:

export default class Foo {}

compiles into:

class Foo {
}
exports.Foo = Foo;

[mhegazy]

@weswigham can you take a look.