Add integrity to link preload tags

Note that the test case isn't very useful as an automatic test because
behaviour differs between browser models and versions, and the test is
only run in a single Chromium version via Puppeteer. The test is still
useful when run manually, like so:

  npm install  # install requirements
  npm install webpack@4 \
    extract-text-webpack-plugin@4.0.0-alpha.0 \
    html-webpack-plugin@3  # upgrade to Webpack 4
  npm run mocha  # Run mocha tests
  (cd examples/webpack-preload/dist/ && python -m SimpleHTTPServer)
  # Visit http://localhost:8000/ with different browsers

See #111

examples/webpack-preload/README.md

@@ -0,0 +1,3 @@
+# With `/* webpackPreload: "true" */`
+
+https://github.com/waysact/webpack-subresource-integrity/issues/111

examples/webpack-preload/index.js

@@ -0,0 +1 @@
+import('./lazy-chunk-1.js');

examples/webpack-preload/lazy-chunk-1.js

@@ -0,0 +1 @@
+import(/* webpackPreload: true */ './lazy-chunk-2.js');

examples/webpack-preload/lazy-chunk-2.js

@@ -0,0 +1 @@
+console.log('ok');

examples/webpack-preload/test.js

@@ -0,0 +1,8 @@
+var webpackVersion = Number(
+  require('webpack/package.json').version.split('.')[0]
+);
+
+module.exports.skip = function skip() {
+  // webpack-preload tag breaks Webpack 1
+  return webpackVersion < 2;
+};

examples/webpack-preload/webpack.config.js

@@ -0,0 +1,18 @@
+var SriPlugin = require('webpack-subresource-integrity');
+var HtmlWebpackPlugin = require('html-webpack-plugin');
+
+module.exports = {
+  entry: {
+    index: './index.js'
+  },
+  output: {
+    crossOriginLoading: 'anonymous'
+  },
+  plugins: [
+    new SriPlugin({
+      hashFuncNames: ['sha256', 'sha384'],
+      enabled: true
+    }),
+    new HtmlWebpackPlugin()
+  ]
+};

jmtp.js

@@ -43,6 +43,24 @@ WebIntegrityJsonpMainTemplatePlugin.prototype.addSriHashes =
     return source;
   };
 
+/*
+ *  Patch jsonp-script code to add the integrity attribute.
+ */
+WebIntegrityJsonpMainTemplatePlugin.prototype.linkPreloadPlugin =
+  function linkPreloadPlugin(mainTemplate, source) {
+    if (!mainTemplate.outputOptions.crossOriginLoading) {
+      this.sriPlugin.errorOnce(
+        this.compilation,
+        'webpack option output.crossOriginLoading not set, code splitting will not work!'
+      );
+    }
+    return (Template.asString || mainTemplate.asString)([
+      source,
+      'link.integrity = sriHashes[chunkId];',
+      'link.crossOrigin = ' + JSON.stringify(mainTemplate.outputOptions.crossOriginLoading) + ';',
+    ]);
+  };
+
 /*
  *  Patch jsonp-script code to add the integrity attribute.
  */
@@ -65,13 +83,15 @@ WebIntegrityJsonpMainTemplatePlugin.prototype.apply = function apply(
   mainTemplate
 ) {
   var jsonpScriptPlugin = this.jsonpScriptPlugin.bind(this, mainTemplate);
+  var linkPreloadPlugin = this.linkPreloadPlugin.bind(this, mainTemplate);
   var addSriHashes = this.addSriHashes.bind(this, mainTemplate);
 
   if (!mainTemplate.hooks) {
     mainTemplate.plugin('jsonp-script', jsonpScriptPlugin);
     mainTemplate.plugin('local-vars', addSriHashes);
   } else if (mainTemplate.hooks.jsonpScript && mainTemplate.hooks.localVars) {
     mainTemplate.hooks.jsonpScript.tap('SriPlugin', jsonpScriptPlugin);
+    mainTemplate.hooks.linkPreload.tap('SriPlugin', linkPreloadPlugin);
     mainTemplate.hooks.localVars.tap('SriPlugin', addSriHashes);
   } else {
     this.sriPlugin.warnOnce(