Rewrite Hub as interface

[yongxu]

The current babel-traverse implementation of Hub will cause bugs if this.hub.file does not exist, which is only injected by File class in babel-core.

This means babel-traverse package can not be used without babel-core unless we monkey-patch hub with a fake file object. The Hub class itself doesn't carry any useful information besides the file reference.

Defining hub as an interface instead of an object, will make babel-traverse a standalone package and resolve issues #4640 #4413

Solution

we can redefine Hub(or a better name maybe) as

// all the methods should be optional(or make hub optional?), so there will be no exception
declare interface Hub {
  mark(type: string, message: string): void;
  getCode(): string;
  getFileScope(): Scope; // or just getFile, maybe?
  onError(node: Object, msg: string, Error: typeof Error = SyntaxError): void;
}

This interface would be sufficient for the current version, and can be extended in the future.
It also can be checked by flowtype

Here are the whys, where Hub appears

// this function is only used in debugging
  mark(type: string, message: string) {
    this.hub.file.metadata.marked.push({
      type,
      message,
      loc: this.node.loc
    });
  }

// this can be replaced by getCode()
export function getSource() {
  let node = this.node;
  if (node.end) {
    return this.hub.file.code.slice(node.start, node.end);
  } else {
    return "";
  }
}

// JSX plugin
  let visitor = {
    JSXOpeningElement(path, state) {
//..........................................
          path.hub.file.scope.push({id: fileNameIdentifier, init: t.stringLiteral(fileName)});

// and called buildCodeFrameError in few places
throw this.hub.file.buildCodeFrameError

Please let me know if you have any suggestion, or better improvement! I will make a PR if I can get some positive faceback :)

more about hub

the current implementation of hub itself only consist 6 lines of code, and the options has never been used anywhere:

export default class Hub {
  constructor(file, options) {
    this.file = file;
    this.options = options;
  }
}

Here are all the places hub appears, when searching keyword hub in package/:

[yongxu]

yongxu@459b5dc
The potential PR for this issue. PR not submitted yet, cuz not tested yet.
Going to take a nap first :)