Fix "require" name override breaking sourcemaps

In order to avoid conflicts with any existing global `require` functions
on the host page, JS bundles are configured to export their `require`
function as `hypothesisRequire` and bundles loader later are modified to
use that function to import modules from earlier bundles.

Overriding the name used for importing from earlier bundles was
previously done by prepending some code to the start of the bundle.
Unfortunately the source map information was not updated so this broke
sourcemap locations in the generated bundle.

Change the approach instead to modify the "prelude" script that
Browserify inserts at the top of the bundle.

scripts/gulp/create-bundle.js

@@ -13,7 +13,6 @@ const exorcist = require('exorcist');
 const envify = require('loose-envify/custom');
 const gulpUtil = require('gulp-util');
 const mkdirp = require('mkdirp');
-const through = require('through2');
 const watchify = require('watchify');
 
 const minifyStream = require('./minify-stream');
@@ -31,54 +30,6 @@ function waitForever() {
   return new Promise(function() {});
 }
 
-/**
- * Create a transform stream which wraps code from the input with a function
- * which is immediately executed (aka. an "IIFE").
- *
- * @param {string} headerCode - Code added at the start of the wrapper function.
- * @param {string} trailerCode - Code added at the end of the wrapper function.
- * @return {Transform} - A Node `Transform` stream.
- */
-function wrapCodeWithFunction(headerCode, trailerCode = '') {
-  const iifeStart = '(function() {' + headerCode + ';';
-  const iifeEnd = ';' + trailerCode + '})()';
-
-  let isFirstChunk = true;
-  return through(
-    function(data, enc, callback) {
-      if (isFirstChunk) {
-        isFirstChunk = false;
-        this.push(Buffer.from(iifeStart));
-      }
-      this.push(data);
-      callback();
-    },
-    function(callback) {
-      this.push(Buffer.from(iifeEnd));
-      callback();
-    }
-  );
-}
-
-/**
- * Wrap a Browserify bundle's code to change the name of the `require` function
- * which the bundle uses to load modules defined in other bundles.
- *
- * Use this together with Browserify's `externalRequireName` option to define/use
- * a different name for the `require` function. This is useful to avoid conflicts
- * with other code on the page which define/use "require".
- *
- * @param {string} name - Replacement name for the `require` function.
- * @return {Transform} - A node `Transform` stream.
- */
-function useExternalRequireName(name) {
-  // Make the `require` lookup inside the bundle find `name` in the global
-  // scope exported by a previous bundle, instead of `require`.
-  return wrapCodeWithFunction(
-    `var require=("function"==typeof ${name}&&${name})`
-  );
-}
-
 /**
  * type Transform = 'coffee';
  *
@@ -176,6 +127,24 @@ module.exports = function createBundle(config, buildOpts) {
     }
   });
 
+  // Override the "prelude" script which Browserify adds at the start of
+  // generated bundles to look for the custom require name set by previous bundles
+  // on the page instead of the default "require".
+  const defaultPreludePath = require.resolve('browser-pack/prelude');
+  const prelude = fs
+    .readFileSync(defaultPreludePath)
+    .toString()
+    .replace(
+      'var previousRequire = typeof require == "function" && require;',
+      `var previousRequire = typeof ${externalRequireName} == "function" && ${externalRequireName};`
+    );
+  if (!prelude.includes(externalRequireName)) {
+    throw new Error(
+      'Failed to modify prelude to use custom name for "require" function'
+    );
+  }
+  bundleOpts.prelude = prelude;
+
   const name = config.name;
 
   const bundleFileName = name + '.bundle.js';
@@ -243,11 +212,10 @@ module.exports = function createBundle(config, buildOpts) {
 
   function build() {
     const output = fs.createWriteStream(bundlePath);
-    const b = bundle.bundle();
-    b.on('error', function(err) {
+    let stream = bundle.bundle();
+    stream.on('error', function(err) {
       log('Build error', err.toString());
     });
-    let stream = b.pipe(useExternalRequireName(externalRequireName));
 
     if (config.minify) {
       stream = stream.pipe(minifyStream());