Add support for .finally in promises (#3)

See: https://stackoverflow.com/a/54746036/11091039

We use `Promise.finally` in core, but the `tsconfig.json` doesn't list it as supported.

js-packages/tsconfig/tsconfig.json

@@ -16,7 +16,7 @@
     "target": "es6",
     "jsx": "preserve",
     "allowJs": true,
-    "lib": ["es2015", "es2017", "dom"],
+    "lib": ["es2015", "es2017", "dom", "es2018.promise"],
     "allowSyntheticDefaultImports": true
   }
 }