diff --git a/packages/metro-bundler/src/ModuleGraph/worker.js b/packages/metro-bundler/src/ModuleGraph/worker.js
index 16ee3cf6..906f39fd 100644
--- a/packages/metro-bundler/src/ModuleGraph/worker.js
+++ b/packages/metro-bundler/src/ModuleGraph/worker.js
@@ -10,20 +10,15 @@
  */
 'use strict';
 
-const asyncify: Asyncify = require('async/asyncify');
 const optimizeModule = require('./worker/optimize-module');
 const transformModule = require('./worker/transform-module');
 const wrapWorkerFn = require('./worker/wrap-worker-fn');
 
-import type {Callback} from './types.flow';
 import type {OptimizationOptions} from './worker/optimize-module';
 import type {TransformOptions} from './worker/transform-module';
 import type {WorkerFnWithIO} from './worker/wrap-worker-fn';
 
-type Asyncify = <A, B, C>((A, B) => C) => (A, B, Callback<C>) => void;
-
-
 exports.optimizeModule =
-  (wrapWorkerFn(asyncify(optimizeModule)): WorkerFnWithIO<OptimizationOptions>);
+  (wrapWorkerFn(optimizeModule): WorkerFnWithIO<OptimizationOptions>);
 exports.transformModule =
-  (wrapWorkerFn(asyncify(transformModule)): WorkerFnWithIO<TransformOptions>);
+  (wrapWorkerFn(transformModule): WorkerFnWithIO<TransformOptions>);
diff --git a/packages/metro-bundler/src/ModuleGraph/worker/__tests__/wrap-worker-fn-test.js b/packages/metro-bundler/src/ModuleGraph/worker/__tests__/wrap-worker-fn-test.js
index 1989e630..a0329ce3 100644
--- a/packages/metro-bundler/src/ModuleGraph/worker/__tests__/wrap-worker-fn-test.js
+++ b/packages/metro-bundler/src/ModuleGraph/worker/__tests__/wrap-worker-fn-test.js
@@ -27,72 +27,71 @@ describe('wrapWorkerFn:', () => {
   let workerFn, wrapped;
   beforeEach(() => {
     workerFn = fn();
-    workerFn.stub.yields();
     wrapped = wrapWorkerFn(workerFn);
   });
 
   const fs = require('fs');
   const mkdirp = require('mkdirp');
 
-  it('reads the passed-in file synchronously as buffer', done => {
-    wrapped(infile, outfile, {}, () => {
-      expect(fs.readFileSync).toBeCalledWith(infile);
-      done();
-    });
+  it('reads the passed-in file synchronously as buffer', () => {
+    wrapped(infile, outfile, {});
+    expect(fs.readFileSync).toBeCalledWith(infile);
   });
 
-  it('calls the worker function with file contents and options', done => {
+  it('calls the worker function with file contents and options', () => {
     const contents = 'arbitrary(contents);';
     const options = {arbitrary: 'options'};
     fs.readFileSync.mockReturnValue(contents);
-    wrapped(infile, outfile, options, () => {
-      expect(workerFn).toBeCalledWith(contents, options, any(Function));
-      done();
-    });
+    wrapped(infile, outfile, options);
+    expect(workerFn).toBeCalledWith(contents, options);
   });
 
-  it('passes through any error that the worker function calls back with', done => {
+  it('passes through any error that the worker function calls back with', () => {
     const error = new Error();
-    workerFn.stub.yields(error);
-    wrapped(infile, outfile, {}, e => {
+    workerFn.stub.throws(error);
+    try {
+      wrapped(infile, outfile, {});
+      throw new Error('should not reach');
+    } catch (e) {
       expect(e).toBe(error);
-      done();
-    });
+    }
   });
 
-  it('writes the result to disk', done => {
+  it('writes the result to disk', () => {
     const result = {arbitrary: 'result'};
-    workerFn.stub.yields(null, result);
-    wrapped(infile, outfile, {}, () => {
-      expect(mkdirp.sync).toBeCalledWith(dirname(outfile));
-      expect(fs.writeFileSync).toBeCalledWith(
-        outfile,
-        JSON.stringify(result),
-        'utf8',
-      );
-      done();
-    });
+    workerFn.stub.returns(result);
+    wrapped(infile, outfile, {});
+    expect(mkdirp.sync).toBeCalledWith(dirname(outfile));
+    expect(fs.writeFileSync).toBeCalledWith(
+      outfile,
+      JSON.stringify(result),
+      'utf8',
+    );
   });
 
-  it('calls back with any error thrown by `mkdirp.sync`', done => {
+  it('calls back with any error thrown by `mkdirp.sync`', () => {
     const error = new Error();
     mkdirp.sync.mockImplementationOnce(() => {
       throw error;
     });
-    wrapped(infile, outfile, {}, e => {
+    try {
+      wrapped(infile, outfile, {});
+      throw new Error('should not reach');
+    } catch (e) {
       expect(e).toBe(error);
-      done();
-    });
+    }
   });
 
-  it('calls back with any error thrown by `fs.writeFileSync`', done => {
+  it('calls back with any error thrown by `fs.writeFileSync`', () => {
     const error = new Error();
     fs.writeFileSync.mockImplementationOnce(() => {
       throw error;
     });
-    wrapped(infile, outfile, {}, e => {
+    try {
+      wrapped(infile, outfile, {});
+      throw new Error('should not reach');
+    } catch (e) {
       expect(e).toBe(error);
-      done();
-    });
+    }
   });
 });
diff --git a/packages/metro-bundler/src/ModuleGraph/worker/wrap-worker-fn.js b/packages/metro-bundler/src/ModuleGraph/worker/wrap-worker-fn.js
index 0b43d779..732bb774 100644
--- a/packages/metro-bundler/src/ModuleGraph/worker/wrap-worker-fn.js
+++ b/packages/metro-bundler/src/ModuleGraph/worker/wrap-worker-fn.js
@@ -17,47 +17,22 @@ const mkdirp = require('mkdirp');
 
 const {dirname} = require('path');
 
-import type {Callback} from '../types.flow';
-
 type Path = string;
-type WorkerFn<Options> = (
-  fileContents: Buffer,
-  options: Options,
-  callback: Callback<Object>,
-) => void;
+type WorkerFn<Options> = (fileContents: Buffer, options: Options) => mixed;
 export type WorkerFnWithIO<Options> = (
   infile: Path,
   outfile: Path,
   options: Options,
-  callback: Callback<>,
 ) => void;
 
 function wrapWorkerFn<Options>(
   workerFunction: WorkerFn<Options>,
 ): WorkerFnWithIO<Options> {
-  return (
-    infile: Path,
-    outfile: Path,
-    options: Options,
-    callback: Callback<>,
-  ) => {
+  return (infile: Path, outfile: Path, options: Options) => {
     const contents = fs.readFileSync(infile);
-    workerFunction(contents, options, (error, result) => {
-      if (error) {
-        callback(error);
-        return;
-      }
-
-      try {
-        mkdirp.sync(dirname(outfile));
-        fs.writeFileSync(outfile, JSON.stringify(result), 'utf8');
-      } catch (writeError) {
-        callback(writeError);
-        return;
-      }
-
-      callback(null);
-    });
+    const result = workerFunction(contents, options);
+    mkdirp.sync(dirname(outfile));
+    fs.writeFileSync(outfile, JSON.stringify(result), 'utf8');
   };
 }