support FIREBASE_DATABASE_EMULATOR_HOST env var (#2005)

* support FIREBASE_DATABASE_EMULATOR_HOST environment variable

* [AUTOMATED]: Prettier Code Styling

* use one env var definition, remove extra logic

* [AUTOMATED]: Prettier Code Styling

* nit: remove blank line

* minor version bump and environment variable guard

* [AUTOMATED]: Prettier Code Styling

* fix unbound variable reference

* don't export new env variable

* [AUTOMATED]: Prettier Code Styling

* revert minor version bump change

* add cross-referencing comment, prefer env var over app options

Author: jmwski

packages/database/src/core/RepoManager.ts

@@ -28,6 +28,16 @@ import { RepoInfo } from './RepoInfo';
 /** @const {string} */
 const DATABASE_URL_OPTION = 'databaseURL';
 
+/**
+ * This variable is also defined in the firebase node.js admin SDK. Before
+ * modifying this definition, consult the definition in:
+ *
+ * https://github.com/firebase/firebase-admin-node
+ *
+ * and make sure the two are consistent.
+ */
+const FIREBASE_DATABASE_EMULATOR_HOST_VAR = 'FIREBASE_DATABASE_EMULATOR_HOST';
+
 let _staticInstance: RepoManager;
 
 /**
@@ -80,7 +90,12 @@ export class RepoManager {
    * @return {!Database}
    */
   databaseFromApp(app: FirebaseApp, url?: string): Database {
-    const dbUrl: string = url || app.options[DATABASE_URL_OPTION];
+    let dbEmulatorHost = process.env[FIREBASE_DATABASE_EMULATOR_HOST_VAR];
+    if (dbEmulatorHost) {
+      dbEmulatorHost = `http://${dbEmulatorHost}`;
+    }
+    const dbUrl: string =
+      url || dbEmulatorHost || app.options[DATABASE_URL_OPTION];
     if (dbUrl === undefined) {
       fatal(
         "Can't determine Firebase Database URL.  Be sure to include " +