Breaking: parse export default function () {} as a FunctionDeclaration with null id

[hzoo]

Ref estree/estree#98 (comment)
Previous discussion: #208

The consensus for ESTree is to make export default function () {} parse as a FunctionDeclaration with the id property with a value of null (matches current behavior of Esprima, Espree, Flow, Babylon) rather than FunctionExpression

Espree workaround is https://github.com/eslint/espree/blob/a3442b5949dd249e1a81ce4e014ca5e0f17f732c/espree.js#L197-L202

interface OptFunctionDeclaration <: FunctionDeclaration {
    id: Identifier | null;
}

interface OptClassDeclaration <: ClassDeclaration {
    id: Identifier | null;
}

interface ExportDefaultDeclaration <: ModuleDeclaration {
    type: "ExportDefaultDeclaration";
    declaration: OptFunctionDeclaration | OptClassDeclaration | VariableDeclaration | Expression;
}

[marijnh]

Does attached patch look good to you?

I guess we'll have to release it as a new major version. @RReverser Do you know of any other breaking things we should do before we release 5.0?

[RReverser]

@marijnh Not really. To be honest, I've opposed this one for a while in early stages, as it's quite a breaking change, which caused trouble when happened in Esprima/Babylon, but now it's probably fine to have it just for the sake of compat.

[marijnh]

@RReverser I read the thread, and I agree with your reasoning there, but as you said, the standard seems to have moved on, so I guess we should follow.