Perf: replace LimitChunkCountPlugin with more naive MergeChunksPlugin

Use more naive implementation of `MergeChunksPlugin` to concatenate all chunks in a single `server-main.js` bundle when building for server, in place of `LimitChunkCountPlugin` which seems to have quadratic runtime complexity and appears to be very slow for builds that include hundreds of async chunks (vs linear in the `MergeChunksPlugin` case).

Before:
```
[client] Build completed in 4.15s
[server] Build completed in 40.967s
```

After:
```
[client] Build completed in 4.195s
[server] Build completed in 6.801s
```

fusion-cli-tests/test/e2e/typescript/test.js

@@ -33,7 +33,7 @@ describe('TypeScript app', () => {
 
     browser.close();
     proc.kill('SIGKILL');
-  }, 25000);
+  }, 35000);
 
   test('`fusion dev` works', async () => {
     const app = dev(dir);
@@ -49,5 +49,5 @@ describe('TypeScript app', () => {
     t.ok(content.includes('FIXTURE_TYPESCRIPT_LOCAL_PACKAGE_CONTENT'));
 
     await app.teardown();
-  }, 15000);
+  }, 35000);
 });

fusion-cli/build/get-webpack-config.js

@@ -20,6 +20,7 @@ const isEsModule = require('../lib/is-es-module.js');
 const LoaderContextProviderPlugin = require('./plugins/loader-context-provider-plugin.js');
 const ChildCompilationPlugin = require('./plugins/child-compilation-plugin.js');
 const NodeSourcePlugin = require('./plugins/node-source-plugin.js');
+const MergeChunksPlugin = require('./plugins/merge-chunks-plugin.js');
 const {
   chunkIdsLoader,
   fileLoader,
@@ -749,8 +750,7 @@ function getWebpackConfig(opts /*: WebpackConfigOpts */) {
       // Need to provide same API to source node modules in client bundles
       // @see: https://github.com/webpack/webpack/blob/v4.46.0/lib/node/NodeSourcePlugin.js#L21-L36
       target !== 'node' && new NodeSourcePlugin(nodeBuiltins),
-      runtime === 'server' &&
-        new webpack.optimize.LimitChunkCountPlugin({maxChunks: 1}),
+      runtime === 'server' && new MergeChunksPlugin(),
       onBuildEnd
         ? new ProgressBarPlugin({
             callback: (progressBar) => {

fusion-cli/build/plugins/merge-chunks-plugin.js

@@ -0,0 +1,48 @@
+/** Copyright (c) 2018 Uber Technologies, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+/*eslint-env node */
+
+const PLUGIN_NAME = 'MergeChunksPlugin';
+
+class MergeChunksPlugin {
+  apply(compiler /*: Object */) {
+    compiler.hooks.compilation.tap(PLUGIN_NAME, (compilation) => {
+      compilation.hooks.optimizeChunks.tap(
+        {
+          name: PLUGIN_NAME,
+          // @see https://github.com/webpack/webpack/blob/9fcaa243573005d6fdece9a3f8d89a0e8b399613/lib/OptimizationStages.js#L10
+          stage: 10,
+        },
+        () => {
+          if (compilation.chunks.size < 2) {
+            return;
+          }
+
+          const chunkGraph = compilation.chunkGraph;
+          const chunks = Array.from(compilation.chunks);
+
+          const rootChunk = chunks[0];
+          for (let i = 1, len = chunks.length; i < len; i++) {
+            const chunk = chunks[i];
+            if (!chunkGraph.canChunksBeIntegrated(rootChunk, chunk)) {
+              throw new Error(
+                'Expected to be able to merge all chunks into one'
+              );
+            }
+
+            chunkGraph.integrateChunks(rootChunk, chunk);
+            compilation.chunks.delete(chunk);
+          }
+        }
+      );
+    });
+  }
+}
+
+module.exports = MergeChunksPlugin;

fusion-plugin-rpc-redux-react/test/utils.js

@@ -90,7 +90,7 @@ export const Runtime = class Runtime {
 async function untilReady(page, port) {
   let started = false;
   let numTries = 0;
-  while (!started && numTries < 10) {
+  while (!started && numTries < 15) {
     try {
       await page.goto(`http://localhost:${port}`);
       started = true;