Adds support for publicPath to enable serving assets from different…

… locations. (#299)

Summary:
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**Summary**

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This is an updated followup from facebook/metro#280, which we would still need to address the following assumptions about `/assets/`:

https://github.com/facebook/metro/blob/e7deea19001d903a47868bb1fe6456bdc4b585e7/packages/metro/src/Server.js#L332

https://github.com/facebook/metro/blob/e7deea19001d903a47868bb1fe6456bdc4b585e7/packages/metro/src/Server.js#L379

https://github.com/facebook/metro/blob/e7deea19001d903a47868bb1fe6456bdc4b585e7/packages/metro/src/Server.js#L385

As pointed out by gdborton, there isn't currently a way to make this a configurable option. I am not certain how to find a proper workaround for `processRequest`.

We found a temporary solution to have our express app pick up serving the assets from a publicPath as a middleware. But the change in this PR is still necessary to get it working fully.

**Test plan**

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- Will add tests once we figure out a comprehensive solution
Pull Request resolved: facebook/metro#299

Reviewed By: rafeca

Differential Revision: D12939229

Pulled By: mjesun

fbshipit-source-id: 769c23468c5ac434f8319e5e7caaf46dd6453f2d

jest/preprocessor.js

@@ -59,6 +59,7 @@ module.exports = {
         minify: false,
         platform: '',
         projectRoot: '',
+        publicPath: '/assets',
         retainLines: true,
         sourceType: 'unambiguous', // b7 required. detects module vs script mode
       },