Update toolchain to latest Node LTS, Yarn.

Author: chriskrycho

package.json

@@ -146,6 +146,7 @@
     "proseWrap": "never"
   },
   "toolchain": {
-    "node": "10.15.1"
+    "node": "10.15.3",
+    "yarn": "1.15.2"
   }
-}
+}