diff --git a/local-cli/__mocks__/fs.js b/local-cli/__mocks__/fs.js
index d4cdab42b..db689247e 100644
--- a/local-cli/__mocks__/fs.js
+++ b/local-cli/__mocks__/fs.js
@@ -118,6 +118,33 @@ fs.writeFileSync.mockImplementation((filePath, content, options) => {
   node[path.basename(filePath)] = content;
 });
 
+const openFds = new Map();
+let nextFd = 3;
+
+fs.openSync.mockImplementation((filePath, flags) => {
+  const dirPath = path.dirname(filePath);
+  const node = getToNode(dirPath);
+  if (!isDirNode(node)) {
+    throw fsError('ENOTDIR', 'not a directory: ' + dirPath);
+  }
+  node[path.basename(filePath)] = '';
+  openFds.set(nextFd, {filePath, flags, node});
+  return nextFd++;
+});
+
+fs.writeSync.mockImplementation((fd, str) => {
+  invariant(typeof str === 'string', 'only strings supported');
+  const data = openFds.get(fd);
+  if (data == null || data.flags !== 'w') {
+    throw fsError('EBADF', 'bad file descriptor, write');
+  }
+  data.node[path.basename(data.filePath)] += str;
+});
+
+fs.closeSync.mockImplementation(fd => {
+  openFds.delete(fd);
+});
+
 fs.mkdir.mockImplementation(asyncify(fs.mkdirSync));
 
 fs.mkdirSync.mockImplementation((dirPath, mode) => {
@@ -332,26 +359,34 @@ fs.createReadStream.mockImplementation(filepath => {
   });
 });
 
-fs.createWriteStream.mockImplementation(file => {
+fs.createWriteStream.mockImplementation(filePath => {
   let node;
+  const writeStream = new stream.Writable({
+    write(chunk, encoding, callback) {
+      this.__chunks.push(chunk);
+      callback();
+    },
+    final(callback) {
+      node[path.basename(filePath)] = this.__chunks.join('');
+      callback();
+    },
+  });
+  writeStream.__file = filePath;
+  writeStream.__chunks = [];
+  writeStream.end = jest.fn(writeStream.end);
+  fs.createWriteStream.mock.returned.push(writeStream);
   try {
-    node = getToNode(dirname(file));
-  } finally {
-    if (typeof node === 'object') {
-      const writeStream = new stream.Writable({
-        write(chunk) {
-          this.__chunks.push(chunk);
-        },
-      });
-      writeStream.__file = file;
-      writeStream.__chunks = [];
-      writeStream.end = jest.fn(writeStream.end);
-      fs.createWriteStream.mock.returned.push(writeStream);
-      return writeStream;
-    } else {
-      throw new Error('Cannot open file ' + file);
+    const dirPath = dirname(filePath);
+    node = getToNode(dirPath);
+    if (!isDirNode(node)) {
+      throw fsError('ENOTDIR', 'not a directory: ' + dirPath);
     }
+    // Truncate the file on opening.
+    node[path.basename(filePath)] = '';
+  } catch (error) {
+    process.nextTick(() => writeStream.emit('error', error));
   }
+  return writeStream;
 });
 fs.createWriteStream.mock.returned = [];
 
diff --git a/local-cli/__tests__/fs-mock-test.js b/local-cli/__tests__/fs-mock-test.js
index c390e3cf2..43efc8bce 100644
--- a/local-cli/__tests__/fs-mock-test.js
+++ b/local-cli/__tests__/fs-mock-test.js
@@ -24,6 +24,10 @@ jest.mock('fs');
 const fs = require('fs');
 
 describe('fs mock', () => {
+  beforeEach(() => {
+    (fs: $FlowFixMe).mock.clear();
+  });
+
   describe('writeFileSync()', () => {
     it('stores content correctly', () => {
       fs.writeFileSync('/test', 'foobar', 'utf8');
@@ -55,4 +59,28 @@ describe('fs mock', () => {
       expect(content).toEqual('foobar');
     });
   });
+
+  describe('createWriteStream()', () => {
+    it('writes content', done => {
+      const stream = fs.createWriteStream('/test');
+      stream.write('hello, ');
+      stream.write('world');
+      stream.end('!');
+      process.nextTick(() => {
+        const content = fs.readFileSync('/test', 'utf8');
+        expect(content).toEqual('hello, world!');
+        done();
+      });
+    });
+  });
+
+  describe('writeSync()', () => {
+    it('writes content', () => {
+      const fd = fs.openSync('/test', 'w');
+      fs.writeSync(fd, 'hello, world!');
+      fs.closeSync(fd);
+      const content = fs.readFileSync('/test', 'utf8');
+      expect(content).toEqual('hello, world!');
+    });
+  });
 });