Revert "Use yarn when running inside yarn workspace. (#3997)"

This reverts commit 2c34d5b66e.

packages/react-scripts/scripts/build.js

@@ -46,6 +46,7 @@ const { printBrowsers } = require('react-dev-utils/browsersHelper');
 const measureFileSizesBeforeBuild =
   FileSizeReporter.measureFileSizesBeforeBuild;
 const printFileSizesAfterBuild = FileSizeReporter.printFileSizesAfterBuild;
+const useYarn = fs.existsSync(paths.yarnLockFile);
 
 // These sizes are pretty large. We'll warn for bundles exceeding them.
 const WARN_AFTER_BUNDLE_GZIP_SIZE = 512 * 1024;
@@ -116,7 +117,7 @@ checkBrowsers(paths.appPath)
         publicUrl,
         publicPath,
         buildFolder,
-        paths.useYarn
+        useYarn
       );
       printBrowsers(paths.appPath);
     },

packages/react-scripts/scripts/eject.js

@@ -224,7 +224,7 @@ inquirer
       }
     }
 
-    if (paths.useYarn) {
+    if (fs.existsSync(paths.yarnLockFile)) {
       const windowsCmdFilePath = path.join(
         appPath,
         'node_modules',

packages/react-scripts/scripts/start.js

@@ -29,6 +29,7 @@ if (process.env.SKIP_PREFLIGHT_CHECK !== 'true') {
 }
 // @remove-on-eject-end
 
+const fs = require('fs');
 const chalk = require('chalk');
 const webpack = require('webpack');
 const WebpackDevServer = require('webpack-dev-server');
@@ -45,6 +46,7 @@ const paths = require('../config/paths');
 const config = require('../config/webpack.config.dev');
 const createDevServerConfig = require('../config/webpackDevServer.config');
 
+const useYarn = fs.existsSync(paths.yarnLockFile);
 const isInteractive = process.stdout.isTTY;
 
 // Warn and crash if required files are missing
@@ -91,13 +93,7 @@ checkBrowsers(paths.appPath)
     const appName = require(paths.appPackageJson).name;
     const urls = prepareUrls(protocol, HOST, port);
     // Create a webpack compiler that is configured with custom messages.
-    const compiler = createCompiler(
-      webpack,
-      config,
-      appName,
-      urls,
-      paths.useYarn
-    );
+    const compiler = createCompiler(webpack, config, appName, urls, useYarn);
     // Load proxy config
     const proxySetting = require(paths.appPackageJson).proxy;
     const proxyConfig = prepareProxy(proxySetting, paths.appPublic);