Enable the metro-visualizer.

[gdborton]

@mjesun @rafeca This attempts to enable the metro visualizer, but I'm seeing an issue resolving nullthrows when attempting to load /visualizer

Error:

{ Error: Unable to resolve module `nullthrows` from `/Users/gary_borton/code/metro-sample-app/node_modules/metro-visualizer/src/app/index.js`: Module `nullthrows` does not exist in the Haste module map or in these directories:
  /Users/gary_borton/code/metro-sample-app/node_modules

This might be related to https://github.com/facebook/react-native/issues/4968
To resolve try the following:
  1. Clear watchman watches: `watchman watch-del-all`.
  2. Delete the `node_modules` folder: `rm -rf node_modules && npm install`.
  3. Reset Metro Bundler cache: `rm -rf /tmp/metro-bundler-cache-*` or `npm start -- --reset-cache`.
  4. Remove haste cache: `rm -rf /tmp/haste-map-react-native-packager-*`.
    at ModuleResolver.resolveDependency (/Users/gary_borton/code/metro-sample-app/node_modules/metro/src/node-haste/DependencyGraph/ModuleResolution.js:209:1301)
    at ResolutionRequest.resolveDependency (/Users/gary_borton/code/metro-sample-app/node_modules/metro/src/node-haste/DependencyGraph/ResolutionRequest.js:83:16)
    at DependencyGraph.resolveDependency (/Users/gary_borton/code/metro-sample-app/node_modules/metro/src/node-haste/DependencyGraph.js:222:485)
    at Object.resolve (/Users/gary_borton/code/metro-sample-app/node_modules/metro/src/lib/transformHelpers.js:149:25)
    at dependencies.map.result (/Users/gary_borton/code/metro-sample-app/node_modules/metro/src/DeltaBundler/traverseDependencies.js:316:29)
    at Array.map (<anonymous>)
    at resolveDependencies (/Users/gary_borton/code/metro-sample-app/node_modules/metro/src/DeltaBundler/traverseDependencies.js:312:16)
    at /Users/gary_borton/code/metro-sample-app/node_modules/metro/src/DeltaBundler/traverseDependencies.js:169:33
    at Generator.next (<anonymous>)
    at step (/Users/gary_borton/code/metro-sample-app/node_modules/metro/src/DeltaBundler/traverseDependencies.js:271:307)
  originModulePath: '/Users/gary_borton/code/metro-sample-app/node_modules/metro-visualizer/src/app/index.js',
  targetModuleName: 'nullthrows',
  message: 'Unable to resolve module `nullthrows` from `/Users/gary_borton/code/metro-sample-app/node_modules/metro-visualizer/src/app/index.js`: Module `nullthrows` does not exist in the Haste module map or in these directories:\n  /Users/gary_borton/code/metro-sample-app/node_modules\n\nThis might be related to https://github.com/facebook/react-native/issues/4968\nTo resolve try the following:\n  1. Clear watchman watches: `watchman watch-del-all`.\n  2. Delete the `node_modules` folder: `rm -rf node_modules && npm install`.\n  3. Reset Metro Bundler cache: `rm -rf /tmp/metro-bundler-cache-*` or `npm start -- --reset-cache`.\n  4. Remove haste cache: `rm -rf /tmp/haste-map-react-native-packager-*`.' }