metro-bundler: DependencyGraph-test: proof of concept for proper test cleanup

Summary: The `DependencyGraph-test` hangs forever if run in isolation because the watch mode is not properly ended. I propose we just wrap each test in a function that does the correct thing, and also migrate tests to `async`/`await`.

Reviewed By: cpojer

Differential Revision: D5245477

fbshipit-source-id: ea30c0e637e0c7b85afe4c76c5e985846ae9b243

packages/metro-bundler/src/node-haste/DependencyGraph.js

@@ -220,6 +220,10 @@ class DependencyGraph extends EventEmitter {
     return this._haste;
   }
 
+  end() {
+    this._haste.end();
+  }
+
   /**
    * Returns the module object for the given path.
    */

packages/metro-bundler/src/node-haste/__tests__/DependencyGraph-test.js

@@ -135,7 +135,21 @@ describe('DependencyGraph', function() {
       process.platform = realPlatform;
     });
 
-    it('should get dependencies', function() {
+    /**
+     * When running a test on the dependency graph, watch mode is enabled by
+     * default, so we must end the watcher to ensure the test does not hang up
+     * (regardless if the test passes or fails).
+     */
+    async function processDgraph(options, processor) {
+      const dgraph = await DependencyGraph.load(options);
+      try {
+        await processor(dgraph);
+      } finally {
+        dgraph.end();
+      }
+    }
+
+    it('should get dependencies', async function() {
       var root = '/root';
       setMockFileSystem({
         root: {
@@ -152,14 +166,12 @@ describe('DependencyGraph', function() {
         },
       });
 
-      var dgraph = DependencyGraph.load({
-        ...defaults,
-        roots: [root],
-      });
-      return getOrderedDependenciesAsJSON(
+      const opts = {...defaults, roots: [root]};
+      return await processDgraph(opts, async dgraph => {
+        const deps = await getOrderedDependenciesAsJSON(
           dgraph,
           '/root/index.js',
-      ).then(function(deps) {
+        );
         expect(deps).toEqual([
           {
             id: 'index',