Add emulator utils to Firestore externs.

Author: yuchenshi

packages/firestore/externs.json

@@ -1,5 +1,5 @@
 {
-  "externs" :  [
+  "externs": [
     "node_modules/@types/node/base.d.ts",
     "node_modules/@types/node/globals.d.ts",
     "node_modules/typescript/lib/lib.es5.d.ts",
@@ -26,6 +26,7 @@
     "packages/logger/dist/src/logger.d.ts",
     "packages/webchannel-wrapper/src/index.d.ts",
     "packages/util/dist/src/crypt.d.ts",
+    "packages/util/dist/src/emulator.d.ts",
     "packages/util/dist/src/environment.d.ts",
     "packages/util/dist/src/compat.d.ts",
     "packages/firestore/export.ts",