Switch to using native ES6 Map as much as possible

[IvanGoncharov]

We need to switch as much as possible ObjMap to Map to prevent bugs like this:

const { buildSchema, graphqlSync } = require('./npmDist');

const schema = buildSchema(`type Query { foo: String }`);
const query = `
  query ($__proto__: Boolean!) {
    __typename
    __typename @skip(if: $__proto__)
  }
`;

const vars = JSON.parse('{ "__proto__": true }');
try {
  console.log(graphqlSync(schema, query, null, null, vars));
} catch (e) {
  console.log(e);
}

[markokr]

AFAIK only Object.create(null) gives proto-less instance, Map objects will have proto by default...

[IvanGoncharov]

@markokr For ES6 Map we would use get/set methods so Map object:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map

[markokr]

Aha, makes sense now.

[PiDelport]

Using maps instead of Object.create(null) for results would also help avoid the problems described in #484 / #504.