diff --git a/packages/metro-memory-fs/src/index.js b/packages/metro-memory-fs/src/index.js
index 8fb2eb5d..104b47b2 100644
--- a/packages/metro-memory-fs/src/index.js
+++ b/packages/metro-memory-fs/src/index.js
@@ -88,6 +88,7 @@ const ASYNC_FUNC_NAMES = [
   'readdir',
   'readFile',
   'realpath',
+  'stat',
   'write',
   'writeFile',
 ];
diff --git a/packages/metro/src/Assets/__tests__/Assets-test.js b/packages/metro/src/Assets/__tests__/Assets-test.js
index a3b21118..759e82b4 100644
--- a/packages/metro/src/Assets/__tests__/Assets-test.js
+++ b/packages/metro/src/Assets/__tests__/Assets-test.js
@@ -10,12 +10,14 @@
 
 'use strict';
 
-jest.mock('fs');
+jest.mock('fs', () => new (require('metro-memory-fs'))());
 jest.mock('image-size');
 
 const {getAssetData, getAsset} = require('../');
 const crypto = require('crypto');
 const fs = require('fs');
+const mkdirp = require('mkdirp');
+const path = require('path');
 
 const mockImageWidth = 300;
 const mockImageHeight = 200;
@@ -26,41 +28,35 @@ require('image-size').mockReturnValue({
 });
 
 describe('getAsset', () => {
+  beforeEach(() => {
+    fs.reset();
+    mkdirp.sync('/root/imgs');
+  });
+
   it('should work for the simple case', () => {
-    fs.__setMockFilesystem({
-      root: {
-        imgs: {
-          'b.png': 'b image',
-          'b@2x.png': 'b2 image',
-        },
-      },
-    });
+    writeImages({'b.png': 'b image', 'b@2x.png': 'b2 image'});
 
     return Promise.all([
-      getAsset('imgs/b.png', ['/root']),
-      getAsset('imgs/b@1x.png', ['/root']),
+      getAssetStr('imgs/b.png', ['/root']),
+      getAssetStr('imgs/b@1x.png', ['/root']),
     ]).then(resp => resp.forEach(data => expect(data).toBe('b image')));
   });
 
   it('should work for the simple case with platform ext', async () => {
-    fs.__setMockFilesystem({
-      root: {
-        imgs: {
-          'b.ios.png': 'b ios image',
-          'b.android.png': 'b android image',
-          'c.png': 'c general image',
-          'c.android.png': 'c android image',
-        },
-      },
+    writeImages({
+      'b.ios.png': 'b ios image',
+      'b.android.png': 'b android image',
+      'c.png': 'c general image',
+      'c.android.png': 'c android image',
     });
 
     expect(
       await Promise.all([
-        getAsset('imgs/b.png', ['/root'], 'ios'),
-        getAsset('imgs/b.png', ['/root'], 'android'),
-        getAsset('imgs/c.png', ['/root'], 'android'),
-        getAsset('imgs/c.png', ['/root'], 'ios'),
-        getAsset('imgs/c.png', ['/root']),
+        getAssetStr('imgs/b.png', ['/root'], 'ios'),
+        getAssetStr('imgs/b.png', ['/root'], 'android'),
+        getAssetStr('imgs/c.png', ['/root'], 'android'),
+        getAssetStr('imgs/c.png', ['/root'], 'ios'),
+        getAssetStr('imgs/c.png', ['/root']),
       ]),
     ).toEqual([
       'b ios image',
@@ -72,93 +68,71 @@ describe('getAsset', () => {
   });
 
   it('should work for the simple case with jpg', () => {
-    fs.__setMockFilesystem({
-      root: {
-        imgs: {
-          'b.png': 'png image',
-          'b.jpg': 'jpeg image',
-        },
-      },
+    writeImages({
+      'b.png': 'png image',
+      'b.jpg': 'jpeg image',
     });
 
     return Promise.all([
-      getAsset('imgs/b.jpg', ['/root']),
-      getAsset('imgs/b.png', ['/root']),
+      getAssetStr('imgs/b.jpg', ['/root']),
+      getAssetStr('imgs/b.png', ['/root']),
     ]).then(data => expect(data).toEqual(['jpeg image', 'png image']));
   });
 
   it('should pick the bigger one', async () => {
-    fs.__setMockFilesystem({
-      root: {
-        imgs: {
-          'b@1x.png': 'b1 image',
-          'b@2x.png': 'b2 image',
-          'b@4x.png': 'b4 image',
-          'b@4.5x.png': 'b4.5 image',
-        },
-      },
+    writeImages({
+      'b@1x.png': 'b1 image',
+      'b@2x.png': 'b2 image',
+      'b@4x.png': 'b4 image',
+      'b@4.5x.png': 'b4.5 image',
     });
 
-    expect(await getAsset('imgs/b@3x.png', ['/root'])).toBe('b4 image');
+    expect(await getAssetStr('imgs/b@3x.png', ['/root'])).toBe('b4 image');
   });
 
   it('should pick the bigger one with platform ext', async () => {
-    fs.__setMockFilesystem({
-      root: {
-        imgs: {
-          'b@1x.png': 'b1 image',
-          'b@2x.png': 'b2 image',
-          'b@4x.png': 'b4 image',
-          'b@4.5x.png': 'b4.5 image',
-          'b@1x.ios.png': 'b1 ios image',
-          'b@2x.ios.png': 'b2 ios image',
-          'b@4x.ios.png': 'b4 ios image',
-          'b@4.5x.ios.png': 'b4.5 ios image',
-        },
-      },
+    writeImages({
+      'b@1x.png': 'b1 image',
+      'b@2x.png': 'b2 image',
+      'b@4x.png': 'b4 image',
+      'b@4.5x.png': 'b4.5 image',
+      'b@1x.ios.png': 'b1 ios image',
+      'b@2x.ios.png': 'b2 ios image',
+      'b@4x.ios.png': 'b4 ios image',
+      'b@4.5x.ios.png': 'b4.5 ios image',
     });
 
     expect(
       await Promise.all([
-        getAsset('imgs/b@3x.png', ['/root']),
-        getAsset('imgs/b@3x.png', ['/root'], 'ios'),
+        getAssetStr('imgs/b@3x.png', ['/root']),
+        getAssetStr('imgs/b@3x.png', ['/root'], 'ios'),
       ]),
     ).toEqual(['b4 image', 'b4 ios image']);
   });
 
   it('should support multiple project roots', async () => {
-    fs.__setMockFilesystem({
-      root: {
-        imgs: {
-          'b.png': 'b image',
-        },
-      },
-      root2: {
-        newImages: {
-          imgs: {
-            'b@1x.png': 'b1 image',
-          },
-        },
-      },
-    });
+    writeImages({'b.png': 'b image'});
+    mkdirp.sync('/root2/newImages/imgs');
+    fs.writeFileSync('/root2/newImages/imgs/b@1x.png', 'b1 image');
 
-    expect(await getAsset('newImages/imgs/b.png', ['/root', '/root2'])).toBe(
+    expect(await getAssetStr('newImages/imgs/b.png', ['/root', '/root2'])).toBe(
       'b1 image',
     );
   });
 });
 
 describe('getAssetData', () => {
+  beforeEach(() => {
+    fs.reset();
+    mkdirp.sync('/root/imgs');
+  });
+
   it('should get assetData', () => {
-    fs.__setMockFilesystem({
-      root: {
-        imgs: {
-          'b@1x.png': 'b1 image',
-          'b@2x.png': 'b2 image',
-          'b@4x.png': 'b4 image',
-          'b@4.5x.png': 'b4.5 image',
-        },
-      },
+    writeImages({
+      'b@1x.png': 'b1 image',
+      'b@2x.png': 'b2 image',
+      'b@4x.png': 'b4 image',
+      'b@4.5x.png': 'b4.5 image',
     });
 
     return getAssetData('/root/imgs/b.png', 'imgs/b.png', []).then(data => {
@@ -182,15 +156,11 @@ describe('getAssetData', () => {
   });
 
   it('should get assetData for non-png images', async () => {
-    fs.__setMockFilesystem({
-      root: {
-        imgs: {
-          'b@1x.jpg': 'b1 image',
-          'b@2x.jpg': 'b2 image',
-          'b@4x.jpg': 'b4 image',
-          'b@4.5x.jpg': 'b4.5 image',
-        },
-      },
+    writeImages({
+      'b@1x.jpg': 'b1 image',
+      'b@2x.jpg': 'b2 image',
+      'b@4x.jpg': 'b4 image',
+      'b@4.5x.jpg': 'b4.5 image',
     });
 
     const data = await getAssetData('/root/imgs/b.jpg', 'imgs/b.jpg', []);
@@ -240,14 +210,10 @@ describe('getAssetData', () => {
       {virtual: true},
     );
 
-    fs.__setMockFilesystem({
-      root: {
-        imgs: {
-          'b@1x.png': 'b1 image',
-          'b@2x.png': 'b2 image',
-          'b@3x.png': 'b3 image',
-        },
-      },
+    writeImages({
+      'b@1x.png': 'b1 image',
+      'b@2x.png': 'b2 image',
+      'b@3x.png': 'b3 image',
     });
 
     const data = await getAssetData('/root/imgs/b.png', 'imgs/b.png', [
@@ -275,28 +241,20 @@ describe('getAssetData', () => {
   });
 
   describe('hash:', () => {
-    let mockFS;
-
     beforeEach(() => {
-      mockFS = {
-        root: {
-          imgs: {
-            'b@1x.jpg': 'b1 image',
-            'b@2x.jpg': 'b2 image',
-            'b@4x.jpg': 'b4 image',
-            'b@4.5x.jpg': 'b4.5 image',
-          },
-        },
-      };
-
-      fs.__setMockFilesystem(mockFS);
+      writeImages({
+        'b@1x.jpg': 'b1 image',
+        'b@2x.jpg': 'b2 image',
+        'b@4x.jpg': 'b4 image',
+        'b@4.5x.jpg': 'b4.5 image',
+      });
     });
 
     it('uses the file contents to build the hash', async () => {
       const hash = crypto.createHash('md5');
 
-      for (const name in mockFS.root.imgs) {
-        hash.update(mockFS.root.imgs[name]);
+      for (const name of fs.readdirSync('/root/imgs')) {
+        hash.update(fs.readFileSync(path.join('/root/imgs', name), 'utf8'));
       }
 
       expect(await getAssetData('/root/imgs/b.jpg', 'imgs/b.jpg', [])).toEqual(
@@ -311,10 +269,21 @@ describe('getAssetData', () => {
         [],
       );
 
-      mockFS.root.imgs['b@4x.jpg'] = 'updated data';
+      fs.writeFileSync('/root/imgs/b@4x.jpg', 'updated data');
 
       const data = await getAssetData('/root/imgs/b.jpg', 'imgs/b.jpg', []);
       expect(data.hash).not.toEqual(initialData.hash);
     });
   });
 });
+
+function writeImages(imgMap) {
+  for (const fileName in imgMap) {
+    fs.writeFileSync(path.join('/root/imgs', fileName), imgMap[fileName]);
+  }
+}
+
+async function getAssetStr(...args) {
+  const buffer = await getAsset(...args);
+  return buffer.toString('utf8');
+}