Throw error to warn of mistaken loading of prod + dev React bundles

packages/react/index.js

@@ -1,6 +1,50 @@
 'use strict';
 
+function testMinificationUsedDCE() {
+  if (process.env.NODE_ENV === 'development') {
+    // We expect this method only to be called in production.
+    throw new Error('This is unreachable');
+  }
+  try {
+    // use scoped variable for our initial test, in case
+    // 'top-level' mangling is not enabled.
+    const source = testMinificationUsedDCE.toString();
+    const longVariableName = true;
+    if (longVariableName && source.match(/longVariableName/g).length === 3) {
+      // We are not minified.
+      // This might be a Node environment where DCE is not expected anyway.
+      return;
+    }
+    if (source.match(/toString/g).length !== 2) {
+      // We always look for two matches:
+      // The actual occurence and then the call to 'match'
+      //
+      // We know for a fact the above line exists so there should be 2
+      // matches.
+      // Therefore the browser gave us invalid source.
+      return;
+    }
+    if (source.match(/unreachable/g).length === 2) {
+      // We always look for two matches:
+      // The actual occurence and then the call to 'match'
+
+      // Dead code elimination would have stripped that branch
+      // because it is impossible to reach in production.
+      setTimeout(function() {
+        // Ensure it gets reported to production logging
+        throw new Error(
+          'React is running in production mode, but dead code ' +
+            'elimination has not been applied. Read how to correctly ' +
+            'configure React for production: ' +
+            'https://fburl.com/react-perf-use-the-production-build'
+        );
+      });
+    }
+  } catch (e) {}
+}
+
 if (process.env.NODE_ENV === 'production') {
+  testMinificationUsedDCE();
   module.exports = require('./cjs/react.production.min.js');
 } else {
   module.exports = require('./cjs/react.development.js');