Make a child expect's prototype chain include the parent expect

... so that the expect.it equivalent of subsequently added assertions are
available as childExpect.toWhatever

Author: papandreou

lib/createTopLevelExpect.js

@@ -1663,30 +1663,32 @@ expectPrototype.clone = function () {
 
 expectPrototype.child = function () {
   this._assertTopLevelExpect();
-  const childExpect = createTopLevelExpect({
-    assertions: {},
-    types: [],
-    typeByName: {},
-    output: this.output.clone(),
-    format: this.outputFormat(),
-    installedPlugins: [],
-  });
-  const parent = (childExpect.parent = this);
+  const childExpect = createTopLevelExpect(
+    {
+      assertions: {},
+      types: [],
+      typeByName: {},
+      output: this.output.clone(),
+      format: this.outputFormat(),
+      installedPlugins: [],
+    },
+    this
+  );
 
   childExpect.exportAssertion = function (testDescription, handler) {
-    parent.addAssertion(testDescription, handler, childExpect);
+    childExpect.parent.addAssertion(testDescription, handler, childExpect);
     return this;
   };
   childExpect.exportType = function (type) {
     if (childExpect.getType(type.name) !== type) {
       childExpect.addType(type);
     }
 
-    parent.addType(type, childExpect);
+    childExpect.parent.addType(type, childExpect);
     return this;
   };
   childExpect.exportStyle = function (name, handler, allowRedefinition) {
-    parent.addStyle(
+    childExpect.parent.addStyle(
       name,
       function (...args) {
         const childOutput = childExpect.createOutput(this.format);
@@ -1986,18 +1988,26 @@ Object.defineProperty(expectPrototype, 'argTypes', {
   },
 });
 
-function createTopLevelExpect({
-  assertions = {},
-  typeByName = { any: anyType },
-  types = [anyType],
-  output,
-  format = magicpen.defaultFormat,
-  installedPlugins = [],
-} = {}) {
+function createTopLevelExpect(
+  {
+    assertions = {},
+    typeByName = { any: anyType },
+    types = [anyType],
+    output,
+    format = magicpen.defaultFormat,
+    installedPlugins = [],
+  } = {},
+  parentExpect
+) {
   const expect = function (...args) {
     return expect._expect(new Context(), args);
   };
-  utils.setPrototypeOfOrExtend(expect, expectPrototype);
+  if (parentExpect) {
+    expect.parent = parentExpect;
+    utils.setPrototypeOfOrExtend(expect, parentExpect);
+  } else {
+    utils.setPrototypeOfOrExtend(expect, expectPrototype);
+  }
 
   if (!output) {
     output = magicpen();
@@ -2007,6 +2017,7 @@ function createTopLevelExpect({
   return extend(expect, {
     _topLevelExpect: expect,
     _outputFormat: format,
+    _frozen: false,
     assertions,
     typeByName,
     installedPlugins,