Bug 1605231 - update Yarn to 1.21.1. r=dmose

Differential Revision: https://phabricator.services.mozilla.com/D57880

--HG--
extra : moz-landing-system : lando

package-lock.json

@@ -2012,9 +2012,10 @@
       }
     },
     "yarn": {
-      "version": "1.17.3",
-      "resolved": "https://registry.npmjs.org/yarn/-/yarn-1.17.3.tgz",
-      "integrity": "sha512-CgA8o7nRZaQvmeF/WBx2FC7f9W/0X59T2IaLYqgMo6637wfp5mMEsM3YXoJtKUspnpmDJKl/gGFhnqS+sON7hA=="
+      "version": "1.21.1",
+      "resolved": "https://registry.npmjs.org/yarn/-/yarn-1.21.1.tgz",
+      "integrity": "sha512-dQgmJv676X/NQczpbiDtc2hsE/pppGDJAzwlRiADMTvFzYbdxPj2WO4PcNyriSt2c4jsCMpt8UFRKHUozt21GQ==",
+      "dev": true
     }
   }
 }

package.json

@@ -3,9 +3,7 @@
   "description": "This package file is for node modules used in mozilla-central",
   "repository": {},
   "license": "MPL-2.0",
-  "dependencies": {
-    "yarn": "^1.16.0"
-  },
+  "dependencies": {},
   "devDependencies": {
     "babel-eslint": "10.0.3",
     "eslint": "6.2.2",
@@ -23,7 +21,8 @@
     "eslint-plugin-prettier": "3.1.0",
     "eslint-plugin-react": "7.14.3",
     "eslint-plugin-spidermonkey-js": "file:tools/lint/eslint/eslint-plugin-spidermonkey-js",
-    "prettier": "1.17.0"
+    "prettier": "1.17.0",
+    "yarn": "1.21.1"
   },
   "notes(private)": "We don't want to publish to npm, so this is marked as private",
   "private": true