Add Scratch.fetch, Scratch.openWindow, and Scratch.redirect stubs in sandboxed extensions

Also includes the appropriate can[Fetch|OpenWindow|Redirect] methods.
Fetching is always allowed. The other methods are always rejected.
The Worker-based sandbox does not have a window.open method and the
iframe-based sandbox has a feature policy that should prevent them from
opening windows already.

src/extension-support/extension-worker.js

@@ -69,7 +69,14 @@ class ExtensionWorker {
 }
 
 global.Scratch = global.Scratch || {};
-Object.assign(global.Scratch, ScratchCommon);
+Object.assign(global.Scratch, ScratchCommon, {
+    canFetch: () => Promise.resolve(true),
+    fetch: (url, options) => fetch(url, options),
+    canOpenWindow: () => Promise.resolve(false),
+    openWindow: () => Promise.reject(new Error('Scratch.openWindow not supported in sandboxed extensions')),
+    canRedirect: () => Promise.resolve(false),
+    redirect: () => Promise.reject(new Error('Scratch.redirect not supported in sandboxed extensions'))
+});
 
 /**
  * Expose only specific parts of the worker to extensions.

test/unit/tw_sandboxed_extensions.js

@@ -0,0 +1,58 @@
+const {test} = require('tap');
+
+/* globals Request */
+global.Request = class {
+    constructor (url) {
+        this.url = url;
+    }
+};
+global.fetch = (url, options = {}) => (
+    Promise.resolve(`[Response ${url instanceof Request ? url.url : url} options=${JSON.stringify(options)}]`)
+);
+
+// Need to trick the extension API to think it's running in a worker
+// It will not actually use this object ever.
+global.self = {};
+// This will install extension worker APIs onto `global`
+require('../../src/extension-support/extension-worker');
+
+test('basic API', t => {
+    t.type(global.Scratch.extensions.register, 'function');
+    t.equal(global.Scratch.ArgumentType.BOOLEAN, 'Boolean');
+    t.equal(global.Scratch.BlockType.REPORTER, 'reporter');
+    t.end();
+});
+
+test('not unsandboxed', t => {
+    t.not(global.Scratch.extensions.unsandboxed, true);
+    t.end();
+});
+
+test('Cast', t => {
+    // Cast is thoroughly tested elsewhere
+    t.equal(global.Scratch.Cast.toString(5), '5');
+    t.equal(global.Scratch.Cast.toNumber(' 5'), 5);
+    t.equal(global.Scratch.Cast.toBoolean('true'), true);
+    t.end();
+});
+
+test('fetch', async t => {
+    t.equal(await global.Scratch.canFetch('https://untrusted.example/'), true);
+    t.equal(await global.Scratch.fetch('https://untrusted.example/'), '[Response https://untrusted.example/ options={}]');
+    t.equal(await global.Scratch.fetch('https://untrusted.example/', {
+        method: 'POST'
+    }), `[Response https://untrusted.example/ options={"method":"POST"}]`);
+    t.end();
+});
+
+test('openWindow', async t => {
+    t.equal(await global.Scratch.canOpenWindow('https://example.com/'), false);
+    await t.rejects(global.Scratch.openWindow('https://example.com/'), /^Scratch\.openWindow not supported in sandboxed extensions$/);
+    t.end();
+});
+
+test('redirect', async t => {
+    t.equal(await global.Scratch.canRedirect('https://example.com/'), false);
+    await t.rejects(global.Scratch.redirect('https://example.com/'), /^Scratch\.redirect not supported in sandboxed extensions$/);
+    t.end();
+});