expose TerminalReporter for Reporter implementations (facebook#1009)

Summary:
We are experimenting with creating our own `Reporter` that can intercept `console.log` messages and turn those into a filterable stream that gets subscribed via a WebSocket. IE

```ts
// this is ok I guess:
import { TerminalReporter } from 'metro/src/lib/TerminalReporter';
// this is preferred, "official" export, hence this PR
import { TerminalReporter } from 'metro';

export default class Reporter extends TerminalReporter {
  update(event) {
    switch (event.type) {
      case 'client_log':
        // send
        break;
      default:
        TerminalReporter.prototype.update(event);
    }
  }
}
```

To make this easy, I just want to expose `TerminalReporter` from the module, rather than reach into the file.

Pull Request resolved: facebook#1009

Test Plan: Not much needed here, AFAICT.

Reviewed By: motiz88

Differential Revision: D46842101

Pulled By: robhogan

fbshipit-source-id: 94f302a07992bd8cb007b2f107284e59a2be47c9

packages/metro/src/index.flow.js

@@ -35,6 +35,7 @@ const makeServeCommand = require('./commands/serve');
 const MetroHmrServer = require('./HmrServer');
 const IncrementalBundler = require('./IncrementalBundler');
 const createWebsocketServer = require('./lib/createWebsocketServer');
+const TerminalReporter = require('./lib/TerminalReporter');
 const MetroServer = require('./Server');
 const outputBundle = require('./shared/output/bundle');
 const chalk = require('chalk');
@@ -140,6 +141,7 @@ type BuildCommandOptions = {} | null;
 type ServeCommandOptions = {} | null;
 
 exports.Terminal = Terminal;
+exports.TerminalReporter = TerminalReporter;
 
 export type {MetroConfig};
 

packages/metro/src/lib/TerminalReporter.js

@@ -515,7 +515,7 @@ class TerminalReporter {
 
   /**
    * Single entry point for reporting events. That allows us to implement the
-   * corresponding JSON reporter easily and have a consistent repor∏ting.
+   * corresponding JSON reporter easily and have a consistent reporting.
    */
   update(event: TerminalReportableEvent): void {
     if (event.type === 'bundle_transform_progressed') {

packages/metro/types/index.d.ts

@@ -33,6 +33,7 @@ import type {OutputOptions, RequestOptions} from './shared/types';
 
 export {loadConfig, mergeConfig, resolveConfig} from 'metro-config';
 export {Terminal} from 'metro-core';
+export {TerminalReporter} from './lib/TerminalReporter';
 
 export {HttpServer, HttpsServer};