diff --git a/src/test/mock-modules.js b/src/test/mock-modules.js
index 52d0393b8b..d8bd57f81a 100644
--- a/src/test/mock-modules.js
+++ b/src/test/mock-modules.js
@@ -16,22 +16,95 @@
  * @providesModule mock-modules
  */
 
+var mocks = require("mocks");
+var exportsRegistry = {};
+var hasOwn = exportsRegistry.hasOwnProperty;
+var explicitMockMap = {};
+
+function getMock(exports) {
+  try {
+    return mocks.generateFromMetadata(mocks.getMetadata(exports));
+  } catch (err) {
+    console.warn(err);
+    return exports;
+  }
+}
+
+// This function should be called at the bottom of any module that might
+// need to be mocked, after the final value of module.exports is known.
 exports.register = function(id, module) {
-    // TODO
+  exportsRegistry[id] = {
+    module: module,
+    actual: module.exports,
+    mocked: null // Filled in lazily later.
+  };
+
+  // If doMock or doNotMock was called earlier, before the module was
+  // registered, then the choice should have been recorded in
+  // explicitMockMap. Now that the module is registered, we can finally
+  // fulfill the request.
+  if (hasOwn.call(explicitMockMap, id)) {
+    if (explicitMockMap[id]) {
+      doMock(id);
+    } else {
+      doNotMock(id);
+    }
+  }
+
+  return exports;
 };
 
+function resetEntry(id) {
+  if (hasOwn.call(exportsRegistry, id)) {
+    delete exportsRegistry[id].module.exports;
+    delete exportsRegistry[id];
+  }
+}
+
 exports.dumpCache = function() {
-    require("mocks").clear();
-    return exports;
+  require("mocks").clear();
+
+  // Deleting module.exports will cause the module to be lazily
+  // reevaluated the next time it is required.
+  for (var id in exportsRegistry) {
+    resetEntry(id);
+  }
+
+  return exports;
 };
 
-exports.dontMock = function() {
-    return exports;
-};
+// Call this function to ensure that require(id) returns the actual
+// exports object created by the module.
+function doNotMock(id) {
+  explicitMockMap[id] = false;
 
-exports.mock = function() {
-    return exports;
-};
+  var entry = exportsRegistry[id];
+  if (entry && entry.module && entry.actual) {
+    entry.module.exports = entry.actual;
+  }
+
+  return exports;
+}
+
+// Call this function to ensure that require(id) returns a mock exports
+// object based on the actual exports object created by the module.
+function doMock(id) {
+  explicitMockMap[id] = true;
+
+  var entry = exportsRegistry[id];
+  if (entry && entry.module && entry.actual) {
+    // Because mocking can be expensive, create the mock exports object on
+    // demand, the first time doMock is called.
+    entry.mocked || (entry.mocked = getMock(entry.actual));
+    entry.module.exports = entry.mocked;
+  }
+
+  return exports;
+}
 
 var global = Function("return this")();
 require('test/mock-timers').installMockTimers(global);
+
+// Exported names are different for backwards compatibility.
+exports.dontMock = doNotMock;
+exports.mock = doMock;