feat(yarn): use yarn to install deps if possible

closes #703

lib/commands/new/project-template.js

@@ -1,6 +1,7 @@
 'use strict';
 const ProjectItem = require('../../project-item').ProjectItem;
 const NPM = require('../../package-managers/npm').NPM;
+const Yarn = require('../../package-managers/yarn').Yarn;
 const path = require('path');
 const fs = require('../../file-system');
 const string = require('../../string');
@@ -321,6 +322,11 @@ exports.ProjectTemplate = class {
 };
 
 function installDependencies(ui, workingDirectory, dependencies) {
+  let yarn = new Yarn();
+  if (yarn.isAvailable(workingDirectory)) {
+    return yarn.install([], { cwd: workingDirectory });
+  }
+
   let npm = new NPM();
   let npmOptions = {
     loglevel: 'error',

lib/package-managers/yarn.js

@@ -1,12 +1,20 @@
 'use strict';
 
 const childProcess = require('child_process');
+const npmWhich = require('npm-which');
 
-exports.yarn = class {
-  install(packages) {
+exports.Yarn = class {
+  install(packages, options) {
     return new Promise((resolve, reject) => {
-      let cmd = `yarn add ${packages.join(' ')}`;
-      let options = { cwd: process.cwd() };
+      if (!options) {
+        options = { cwd: process.cwd() };
+      }
+      let yarnPath = this.getYarnPath(options.cwd);
+      let cmd = `"${yarnPath}"`;
+
+      if (packages && packages.length > 0) {
+        cmd += ` add ${packages.join(' ')}`;
+      }
 
       let installProcess = childProcess.exec(cmd, options, (error, stdout, stderr) => {
         if (error) {
@@ -20,6 +28,22 @@ exports.yarn = class {
       installProcess.stdout.on('data', data => console.log(data));
     });
   }
+
+  getYarnPath(directory) {
+    return getPathToExecutable('yarn', directory);
+  }
+
+  isAvailable(directory) {
+    return !!this.getYarnPath(directory);
+  }
 };
 
-exports.default = exports.yarn;
+function getPathToExecutable(executableName, directory) {
+  try {
+    return npmWhich(directory).sync(executableName);
+  } catch (ex) {
+    return null;
+  }
+}
+
+exports.default = exports.Yarn;

package.json

@@ -34,6 +34,7 @@
     "esprima": "^4.0.0",
     "glob": "^7.1.1",
     "npm": "^3.10.8",
+    "npm-which": "^3.0.1",
     "preprocess": "^3.1.0",
     "rfc6902": "^1.2.2",
     "semver": "^5.3.0"