Add yarn as a dev dependency.

Fixes #4459

package.json

@@ -53,7 +53,8 @@
     "eslint": "^4.12.1",
     "lerna": "^2.4.0",
     "npm-run-all": "~4.1.1",
-    "tslint": "^5.8.0"
+    "tslint": "^5.8.0",
+    "yarn": "^1.6.0"
   },
   "workspaces": [
     "dev_mode",

yarn.lock

@@ -8993,6 +8993,10 @@ yargs@~3.10.0:
     decamelize "^1.0.0"
     window-size "0.1.0"
 
+yarn@^1.6.0:
+  version "1.6.0"
+  resolved "https://registry.npmjs.org/yarn/-/yarn-1.6.0.tgz#9cec6f7986dc237d39ec705ce74d95155fe55d4b"
+
 yeast@0.1.2:
   version "0.1.2"
   resolved "https://registry.npmjs.org/yeast/-/yeast-0.1.2.tgz#008e06d8094320c372dbc2f8ed76a0ca6c8ac419"