fix(scopes): Support Function and Class Declarations without id

[danez]

Declarations without id are used in combination with default exports

export default function () {};

http://astexplorer.net/#/gist/cf7df7dd64907912d4c0b0c270c4d42c/39593db1b7ae5b05649f7d524e78f7439359602c

Without the fix I receive:

TypeError: Cannot read property 'type' of null
    at extractNames (/Users/danieltschinder/Documents/Github/babel/node_modules/rollup-pluginutils/dist/pluginutils.cjs.js:52:20)
    at Scope.addDeclaration (/Users/danieltschinder/Documents/Github/babel/node_modules/rollup-pluginutils/dist/pluginutils.cjs.js:86:3)
    at Object.enter (/Users/danieltschinder/Documents/Github/babel/node_modules/rollup-pluginutils/dist/pluginutils.cjs.js:108:11)
    at visit (/Users/danieltschinder/Documents/Github/babel/node_modules/rollup-pluginutils/node_modules/estree-walker/dist/estree-walker.umd.js:32:9)
    at visit (/Users/danieltschinder/Documents/Github/babel/node_modules/rollup-pluginutils/node_modules/estree-walker/dist/estree-walker.umd.js:51:4)
    at visit (/Users/danieltschinder/Documents/Github/babel/node_modules/rollup-pluginutils/node_modules/estree-walker/dist/estree-walker.umd.js:46:5)
    at Object.walk (/Users/danieltschinder/Documents/Github/babel/node_modules/rollup-pluginutils/node_modules/estree-walker/dist/estree-walker.umd.js:11:2)
    at Object.attachScopes (/Users/danieltschinder/Documents/Github/babel/node_modules/rollup-pluginutils/dist/pluginutils.cjs.js:103:15)
    at inject (/Users/danieltschinder/Documents/Github/babel/node_modules/rollup-plugin-node-globals/dist/rollup-plugin-node-globals.cjs.js:83:33)
    at Object.transform (/Users/danieltschinder/Documents/Github/babel/node_modules/rollup-plugin-node-globals/dist/rollup-plugin-node-globals.cjs.js:254:17)

//cc @Rich-Harris, @rollup :)

[danez]

@Rich-Harris I know pinging people directly is annoying, but I would really like to move forward with migrating babel-standalone to rollup and I'm blocked by this. So If you have a free minute I would really appreciate if you could review and if okay merge and release.

[guybedford]

This looks good to me, will see if we can get this merged soon.

[lukastaegert]

Note that by npm owner ls rollup-pluginutils, @Rich-Harris is currently the only person able to release new versions on npm. Maybe this should be extended...?

[Rich-Harris]

Just saw this — @lukastaegert, have added you and Guy as npm owners. Perhaps we should eventually consider moving some of these packages into a @rollup scope