diff --git a/.travis.yml b/.travis.yml
index d50d75f48..04f3fcdf9 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -7,6 +7,7 @@ before_script:
   - sh -e /etc/init.d/xvfb start
   - npm install -g karma
   - npm install -g typescript
+  - npm install -g yarn
   - ./nodeserver.sh > /dev/null &
 
 matrix:
@@ -27,7 +28,7 @@ script:
   - echo "CORE_BRANCH = x${CORE_BRANCH}x";
   - >
     if [ "x${CORE_BRANCH}x" != "xx" ] ; then 
-      git clone --depth=10 --branch="${CORE_BRANCH}" https://github.com/ui-router/core.git ui-router-core && npm install ./ui-router-core ;
+      git clone --depth=10 --branch="${CORE_BRANCH}" https://github.com/ui-router/core.git ui-router-core && pushd ui-router-core && yarn && popd && npm install ./ui-router-core ;
     fi
   - npm test
 
diff --git a/karma.conf.js b/karma.conf.js
index 02674c85b..2ccf3d21c 100644
--- a/karma.conf.js
+++ b/karma.conf.js
@@ -23,15 +23,15 @@ function karmaServedFiles(ngVersion) {
     ];
   }
 
-  let angularFiles = angular(ngVersion).map(function (pattern) {
+  var angularFiles = angular(ngVersion).map(function (pattern) {
     return { watched: false, included: true, nocache: true, pattern: pattern };
   });
 
-  let srcFiles = [
+  var srcFiles = [
     { watched: true, included: false, nocache: true, pattern: 'src/**/*.ts' },
   ];
 
-  let testFiles = [
+  var testFiles = [
     { watched: true, included: false, nocache: true, pattern: 'test/**/*.ts' },
     { watched: true, included: false, nocache: true, pattern: 'test/**/*.js' }
   ];