fix: added readonly function and tests

Author: jhefferman-sfdc

packages/@lwc/engine-server/src/__tests__/fixtures/read-only-props/function/config.json

@@ -0,0 +1,3 @@
+{
+    "entry": "x/component"
+}

packages/@lwc/engine-server/src/__tests__/fixtures/read-only-props/function/error.txt

@@ -0,0 +1,10 @@
+<fixture-test>
+  <template shadowrootmode="open">
+    <div>
+      Is original the same as proxy? false
+    </div>
+    <div>
+      Is original the same as proxy when stringified? true
+    </div>
+  </template>
+</fixture-test>

packages/@lwc/engine-server/src/__tests__/fixtures/read-only-props/function/expected.html

@@ -0,0 +1,4 @@
+<template>
+    <div>Is original the same as proxy? {areSame}</div>
+    <div>Is original the same as proxy when stringified? {areSameStringified}</div>
+</template>

packages/@lwc/engine-server/src/__tests__/fixtures/read-only-props/function/modules/x/component/component.html

@@ -0,0 +1,9 @@
+import { LightningElement, readonly } from 'lwc';
+
+export default class extends LightningElement {
+    foo = { willSays: 'mobs 4eva' };
+    readonlyFoo = readonly(this.foo);
+    // Proxy is the same when stringified but it's a different object
+    areSameStringified = JSON.stringify(this.foo) === JSON.stringify(this.readonlyFoo);
+    areSame = this.foo == this.readonlyFoo;
+}

packages/@lwc/engine-server/src/__tests__/fixtures/read-only-props/function/modules/x/component/component.js

@@ -15,3 +15,14 @@ const reactiveMembrane = new ObservableMembrane();
 export function getReadOnlyProxy<T>(value: T): Readonly<T> {
     return reactiveMembrane.getReadOnlyProxy(value);
 }
+
+/**
+ * DEPRECATED: This function allows you to create a reactive readonly
+ * membrane around any object value.
+ * @param value any object
+ * @returns a readonly proxy of the live object
+ * @deprecated
+ */
+export function readonly<T>(value: T): Readonly<T> {
+    return reactiveMembrane.getReadOnlyProxy(value);
+}

packages/@lwc/ssr-runtime/src/get-read-only-proxy.ts

@@ -40,3 +40,4 @@ export { hasScopedStaticStylesheets, renderStylesheets } from './styles';
 export { toIteratorDirective } from './to-iterator-directive';
 export { validateStyleTextContents } from './validate-style-text-contents';
 export { createContextProvider, establishContextfulRelationship, connectContext } from './wire';
+export { readonly } from './get-read-only-proxy';

packages/@lwc/ssr-runtime/src/index.ts

@@ -30,9 +30,6 @@ export function parseFragment(..._: unknown[]): never {
 export function parseSVGFragment(..._: unknown[]): never {
     throw new Error('parseSVGFragment cannot be used in SSR context.');
 }
-export function readonly(..._: unknown[]): never {
-    throw new Error('readonly cannot be used in SSR context.');
-}
 export function registerComponent(..._: unknown[]): never {
     throw new Error('registerComponent cannot be used in SSR context.');
 }