WIP fix and test 'testMinificationUsedDCE' method

**what is the change?:**
- Instead of looking for one match when having the method inspect it's
  own source, we look for two matches, because the search itself will be
  a match.
- WIP moving this method into another file and testing it.

Next steps:
- Figure out why the babel.transform call is not actually minifying the
  code
- Add tests for uglifyjs
- Update build process so that this file can be accessed from
  `packages/react/index.js`

**why make this change?:**
- We had a bug in the 'testMinification' method, and I thought the name
  could be more clear. I fixed the code and also changed the name.
- In order to avoid other bugs and keep this code working in the future
  we'd like to add a unit test for this method. Using the npm modules
  for 'uglifyjs' and 'babel'/'babili' we should be able to actually test
  how this method will work when minified with different configurations.

**test plan:**
`yarn test`

**issue:**
facebook#9589

packages/react/index.js

@@ -1,38 +1,9 @@
 'use strict';
 
-function testMinification() {
-  if (testMinification.name === 'testMinification') {
-    // We are not minified.
-    // This might be a Node environment where DCE is not expected anyway.
-    return;
-  }
-  if (process.env.NODE_ENV === 'development') {
-    // We expect this method only to be called in production.
-    throw new Error('This is unreachable');
-  }
-  try {
-    const source = testMinification.toString();
-    if (source.indexOf('toString') === -1) {
-      // We know for a fact the above line exists.
-      // Therefore the browser gave us invalid source.
-      return;
-    }
-    if (source.indexOf('unreachable') !== -1) {
-      // 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') {
-  testMinification();
+  // TODO: actually update the build process so this works.
+  (require('./cjs/testMinificationUsedDCE.js'))();
   module.exports = require('./cjs/react.production.min.js');
 } else {
   module.exports = require('./cjs/react.development.js');

src/shared/utils/__tests__/testMinificationUsedDCE-test.js

@@ -0,0 +1,86 @@
+/**
+ * Copyright 2013-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @emails react-core
+ */
+'use strict';
+
+var babel = require('babel-core');
+//var minify = require('babel-preset-minify');
+var testMinificationUsedDCE;
+
+
+describe('in the production environment', () => {
+  let oldProcess;
+  beforeEach(() => {
+    __DEV__ = false;
+
+    // Mutating process.env.NODE_ENV would cause our babel plugins to do the
+    // wrong thing. If you change this, make sure to test with jest --no-cache.
+    oldProcess = process;
+    global.process = {
+      ...process,
+      env: {...process.env, NODE_ENV: 'production'},
+    };
+
+    jest.resetModules();
+    testMinificationUsedDCE = require('testMinificationUsedDCE');
+  });
+
+  afterEach(() => {
+    __DEV__ = true;
+    global.process = oldProcess;
+
+  });
+
+  describe('when not minified', () => {
+    it('should not throw', () => {
+      expect(() => {
+        testMinificationUsedDCE();
+        jest.runAllTimers();
+      }).not.toThrow();
+    });
+  });
+
+  describe('when minified with babel/minify with *no* DCE', () => {
+    xit('should throw', () => {
+      const babelOpts = {
+        presets: ['babel-preset-minify'],
+      };
+      // TODO: Why is this not actually minifying the code????
+      const minifiedWithNoDCE = () => {
+        eval(
+          babel.transform(testMinificationUsedDCE.toString(), babelOpts).code
+        );
+      };
+      expect(() => {
+        minifiedWithNoDCE();
+        jest.runAllTimers();
+      }).toThrow();
+    });
+  });
+
+  describe('when minified with babel/minify with DCE', () => {
+    xit('should not throw', () => {
+      const babelOpts = {
+        plugins: ['minify-dead-code-elimination'],
+        presets: ['babel-preset-minify'],
+      };
+      // TODO: Why is this not actually minifying the code????
+      const minifiedWithDCE = () => {
+        eval(
+          babel.transform(testMinificationUsedDCE.toString(), babelOpts).code
+        );
+      };
+      expect(() => {
+        testMinificationUsedDCE();
+        jest.runAllTimers();
+      }).not.toThrow();
+    });
+  });
+});

src/shared/utils/testMinificationUsedDCE.js

@@ -0,0 +1,56 @@
+/**
+ * Copyright 2014-2015, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @providesModule testMinificationUsedDCE
+ */
+
+'use strict';
+
+function testMinificationUsedDCE() {
+  if (process.env.NODE_ENV === 'production') {
+    const source = testMinificationUsedDCE.toString();
+    const longVariableName = source;
+    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 (process.env.NODE_ENV === 'development') {
+      // We expect this method only to be called in production.
+      throw new Error('This is unreachable');
+    }
+    try {
+      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) {}
+  }
+}
+
+module.exports = testMinificationUsedDCE;