Use yarn when available

Summary:
**Motivation**

`react-native init` takes minutes even on a fast network. Yarn makes it much quicker.

When yarn is not installed on the system:

<img width="440" alt="screenshot 2016-10-31 22 21 19" src="https://cloud.githubusercontent.com/assets/346214/19873897/7bad5662-9fb9-11e6-85fb-ad4879949dad.png">

When yarn is installed:

<img width="441" alt="screenshot 2016-10-31 22 02 20" src="https://cloud.githubusercontent.com/assets/346214/19873898/7baf4c56-9fb9-11e6-96b3-007f93d2438a.png">

Also added the option `react-native init AwesomeApp --npm` as a fallback in case yarn is not stable enough for some people (I saw some Github issues that yarn hangs for example; for me it works great though).

**Test plan**
1. Publish to Sinopia: https://github.com/facebook/react-native/tree/master/react-native-cli
2. `react-native init AwesomeApp`

***Tested the following setups***

- New CLI - uses yarn, new react-native - uses yarn
- Old CLI (1.0.0) - doesn't use yarn, new react-native - uses yarn
-
Closes https://github.com/facebook/react-native/pull/10626

Differential Revision: D4110883

Pulled By: bestander

fbshipit-source-id: 8a3427e2bc9158cf5fadd8ddf5b31e4b50ce6453

local-cli/generator/index.js

@@ -8,10 +8,39 @@
  */
 'use strict';
 
+var execSync = require('child_process').execSync;
 var fs = require('fs');
 var path = require('path');
-var yeoman = require('yeoman-generator');
+var semver = require('semver')
 var utils = require('../generator-utils');
+var yeoman = require('yeoman-generator');
+
+// Use Yarn if available, it's much faster than the npm client.
+// Return the version of yarn installed on the system, null if yarn is not available.
+function getYarnVersionIfAvailable() {
+  let yarnVersion;
+  try {
+    // execSync returns a Buffer -> convert to string
+    if (process.platform.startsWith('win')) {
+      yarnVersion = (execSync('yarn --version').toString() || '').trim();
+    } else {
+      yarnVersion = (execSync('yarn --version 2>/dev/null').toString() || '').trim();
+    }
+  } catch (error) {
+    return null;
+  }
+  // yarn < 0.16 has a 'missing manifest' bug
+  try {
+    if (semver.gte(yarnVersion, '0.16.0')) {
+      return yarnVersion;
+    } else {
+      return null;
+    }
+  } catch (error) {
+    console.error('Cannot parse yarn version: ' + yarnVersion);
+    return null;
+  }
+}
 
 module.exports = yeoman.generators.NamedBase.extend({
   constructor: function() {
@@ -36,6 +65,12 @@ module.exports = yeoman.generators.NamedBase.extend({
       type: Boolean,
       defaults: false
     });
+    // Temporary option until yarn becomes stable.
+    this.option('npm', {
+      desc: 'Use the npm client, even if yarn is available.',
+      type: Boolean,
+      defaults: false
+    });
 
     // this passes command line arguments down to the composed generators
     var args = {args: arguments[0], options: this.options};
@@ -112,12 +147,24 @@ module.exports = yeoman.generators.NamedBase.extend({
       return;
     }
 
-    this.npmInstall(`react@${reactVersion}`, { '--save': true, '--save-exact': true });
+    const yarnVersion = (!this.options['npm']) && getYarnVersionIfAvailable();
+
+    console.log('Installing React...');
+    if (yarnVersion) {
+      execSync(`yarn add react@${reactVersion}`);
+    } else {
+      this.npmInstall(`react@${reactVersion}`, { '--save': true, '--save-exact': true });
+    }
     if (!this.options['skip-jest']) {
-      this.npmInstall(`jest babel-jest jest-react-native babel-preset-react-native react-test-renderer@${reactVersion}`.split(' '), {
-        saveDev: true,
-        '--save-exact': true
-      });
+      console.log('Installing Jest...');
+      if (yarnVersion) {
+        execSync(`yarn add jest babel-jest jest-react-native babel-preset-react-native react-test-renderer@${reactVersion} --dev --exact`);
+      } else {
+        this.npmInstall(`jest babel-jest jest-react-native babel-preset-react-native react-test-renderer@${reactVersion}`.split(' '), {
+          saveDev: true,
+          '--save-exact': true
+        });
+      }
       fs.writeFileSync(
         path.join(
           this.destinationRoot(),
@@ -125,7 +172,6 @@ module.exports = yeoman.generators.NamedBase.extend({
         ),
         '{\n"presets": ["react-native"]\n}'
       );
-
       this.fs.copy(
         this.templatePath('__tests__'),
         this.destinationPath('__tests__'),
@@ -133,7 +179,6 @@ module.exports = yeoman.generators.NamedBase.extend({
           nodir: false
         }
       );
-
       var packageJSONPath = path.join(
         this.destinationRoot(),
         'package.json'