SRI for assets added in beforeHtmlGeneration

This effectively provides compatbility with assets added by plugins
such as add-asset-html-webpack-plugin.

Also adds a test case for DllPlugin with html-webpack-plugin.

Closes #51

Author: jscheid

examples/dll-plugin/README.md

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+# DllPlugin Integration #hwp
+
+Demonstrates how assets generated by DllPlugin can be included with
+SRI using html-webpack-plugin.

examples/dll-plugin/a.js

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+module.exports = 'a';

examples/dll-plugin/alpha.js

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+module.exports = 'alpha';

examples/dll-plugin/b.js

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+module.exports = 'b';

examples/dll-plugin/beta.js

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+module.exports = 'beta';

examples/dll-plugin/c.jsx

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+module.exports = 'ok';

examples/dll-plugin/index.js

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+console.log(require('beta/beta'));
+console.log(require('beta/b'));
+console.log(require('beta/c.jsx'));

examples/dll-plugin/test.js

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+var expect = require('expect');
+var webpackVersion = Number(
+  require('webpack/package.json').version.split('.')[0]
+);
+
+module.exports.skip = function skip() {
+  // Can't use add-asset-html-webpack-plugin with older Webpack versions
+  return webpackVersion < 4;
+};
+
+module.exports.check = function check(stats) {
+  expect(stats.compilation.errors).toEqual([]);
+  expect(stats.compilation.warnings).toEqual([]);
+};

examples/dll-plugin/webpack.config.js

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+var SriPlugin = require('webpack-subresource-integrity');
+var HtmlWebpackPlugin = require('html-webpack-plugin');
+var WebpackBeforeBuildPlugin = require('before-build-webpack');
+var webpack = require('webpack');
+var path = require('path');
+var AddAssetHtmlPlugin = require('add-asset-html-webpack-plugin');
+
+module.exports = {
+  // mode: "development || "production",
+  resolve: {
+    extensions: ['.js', '.jsx']
+  },
+  entry: {
+    alpha: ['./alpha', './a'],
+    beta: ['./beta', './b', './c']
+  },
+  output: {
+    filename: 'MyDll.[name].js',
+    library: '[name]_[hash]'
+  },
+  plugins: [
+    new webpack.DllPlugin({
+      path: path.join(__dirname, 'dist', '[name]-manifest.json'),
+      name: '[name]_[hash]'
+    }),
+    new WebpackBeforeBuildPlugin(
+      function(_stats, callback) {
+        webpack(
+          {
+            mode: 'production',
+            entry: {
+              index: './index.js'
+            },
+            output: {
+              path: path.join(__dirname, 'dist'),
+              crossOriginLoading: 'anonymous'
+            },
+            plugins: [
+              new webpack.DllReferencePlugin({
+                context: path.join(__dirname),
+                manifest: require(path.join(
+                  __dirname,
+                  'dist/alpha-manifest.json'
+                )) // eslint-disable-line
+              }),
+              new webpack.DllReferencePlugin({
+                scope: 'beta',
+                manifest: require(path.join(
+                  __dirname,
+                  'dist/beta-manifest.json'
+                )), // eslint-disable-line
+                extensions: ['.js', '.jsx']
+              }),
+              new HtmlWebpackPlugin(),
+              new AddAssetHtmlPlugin({
+                filepath: path.resolve(__dirname, 'dist/MyDll.*.js')
+              }),
+              new SriPlugin({
+                hashFuncNames: ['sha256', 'sha384'],
+                enabled: true
+              })
+            ]
+          },
+          function afterEmit(err, stats) {
+            if (err || stats.hasErrors() || stats.hasWarnings()) {
+              throw err || new Error(stats.toString({ reason: true }));
+            } else {
+              callback();
+            }
+          }
+        );
+      },
+      ['done']
+    )
+  ]
+};