Yarn as a dev dep and auto fetch and install mobile code

* Yarn is installed locally as a development dependency
* The mobile submodule is updated in preinstall
* The mobile packages are installed in preinstall as well
* Yarn is executed by providing the working (sub)directory as a
parameter
* No need to install yarn globally in Travis

.travis.yml

@@ -26,7 +26,6 @@ before_install:
     }
   - nvm install && nvm use
   - npm install npm -g
-  - npm install yarn -g
 
 branches:
   only:

bin/install-node-nvm.sh

@@ -72,21 +72,6 @@ npm install
 # Make sure npm is up-to-date
 npm install npm -g
 
-# Check if yarn is installed
-if [ "$TRAVIS" != "true" ] && ! command_exists "yarn"; then
-	if ask "$(error_message "Yarn isn't installed, would you like to download and install it automatically?")" Y; then
-		echo -en $(status_message "Installing yarn..." )
-		npm install -g yarn >/dev/null 2>&1
-		echo ' done!'
-	else
-		echo -e $(error_message "")
-		echo -e $(error_message "Please install Yarn manually, then re-run the setup script to continue.")
-		echo -e $(error_message "Yarn installation instructions can be found here: $(action_format "https://yarnpkg.com/lang/en/docs/install/#install-via-npm")")
-	fi
-
-	exit 1
-fi
-
 # There was a bug in NPM that caused changes in package-lock.json. Handle that.
 if [ "$TRAVIS" != "true" ] && ! git diff --exit-code package-lock.json >/dev/null; then
 	if ask "$(warning_message "Your package-lock.json changed, which may mean there's an issue with your NPM cache. Would you like to try and automatically clean it up?" )" N 10; then

package.json

@@ -121,7 +121,8 @@
 		"webpack": "4.8.3",
 		"webpack-cli": "2.1.3",
 		"webpack-livereload-plugin": "2.1.1",
-		"webpack-rtl-plugin": "github:yoavf/webpack-rtl-plugin#develop"
+		"webpack-rtl-plugin": "github:yoavf/webpack-rtl-plugin#develop",
+		"yarn": "1.9.4"
 	},
 	"npmPackageJsonLintConfig": {
 		"extends": "@wordpress/npm-package-json-lint-config",
@@ -175,7 +176,10 @@
 		"lint-css": "stylelint '**/*.scss'",
 		"lint-css:fix": "stylelint '**/*.scss' --fix",
 		"package-plugin": "./bin/build-plugin-zip.sh",
-		"postinstall": "npm run check-licenses && npm run build:packages",
+		"mobile-submodule-update": "git submodule update --init --recursive",
+		"mobile-install": "npx yarn --cwd gutenberg-mobile install",
+		"preinstall": "npm run mobile-submodule-update && npm run mobile-install && npm run check-licenses && npm run build:packages",
+		"postinstall": "npx yarn --cwd gutenberg-mobile install && npm run check-licenses && npm run build:packages",
 		"pot-to-php": "./bin/pot-to-php.js",
 		"precommit": "lint-staged",
 		"publish:check": "npm run build:packages && lerna updated",
@@ -185,7 +189,7 @@
 		"pretest-e2e": "concurrently \"./bin/reset-e2e-tests.sh\" \"npm run build\"",
 		"test-e2e": "cross-env JEST_PUPPETEER_CONFIG=test/e2e/puppeteer.config.js wp-scripts test-unit-js --config test/e2e/jest.config.json --runInBand",
 		"test-e2e:watch": "npm run test-e2e -- --watch",
-		"test-mobile": "cd gutenberg-mobile; yarn test:inside-gb; cd ..;",
+		"test-mobile": "npx yarn --cwd gutenberg-mobile test:inside-gb",
 		"test-php": "npm run lint-php && npm run test-unit-php",
 		"test-unit": "wp-scripts test-unit-js --config test/unit/jest.config.json",
 		"test-unit:coverage": "npm run test-unit -- --coverage",