Add Yarn to the e2e test

Run tests both with and without Yarn installed.

Author: fson

.travis.yml

@@ -9,3 +9,6 @@ cache:
   - packages/create-react-app/node_modules
   - packages/react-scripts/node_modules
 script: tasks/e2e.sh
+env:
+  - USE_YARN=no
+  - USE_YARN=yes

tasks/e2e.sh

@@ -53,6 +53,12 @@ set -x
 cd ..
 root_path=$PWD
 
+if [ "$USE_YARN" = "yes" ]
+then
+  # Install Yarn so that the test can use it to install packages.
+  npm install -g yarn
+fi
+
 npm install
 
 # Lint own code