Implement fallback capability (#44)

* Implement fallback capability

This makes it possible for a dynamic component to render fallback content on the server.

The mechanism is a special `static` prop passed to the component. If `static` is true then the component knows it can render static content.

Putting aside the word `static`, is this the right approach? I think giving components the flexibility to make the decision themselves *is* the right approach.

However in this case we have a special property that is passed in non-explicitly. I think we have to do it this way because if the caller passes in a prop it will get serialized and appear on the client. By making this something we *add* during rendering, it only happens on the server (and only when using `:load`).

Assuming this is the right approach, is `static` the right name for this prop? Other candidates:

* `server`

That's all I have!

* Use `import.meta.env.astro` to tell if running in SSR mode.

* Run formatter

1 files changed, 19 insertions, 0 deletions

diff --git a/test/astro-fallback.test.js b/test/astro-fallback.test.js
new file mode 100644
index 000000000..2acf29f8e
--- /dev/null
+++ b/test/astro-fallback.test.js
@@ -0,0 +1,19 @@
+import { suite } from 'uvu';
+import * as assert from 'uvu/assert';
+import { doc } from './test-utils.js';
+import { setup } from './helpers.js';
+
+const Fallback = suite('Dynamic component fallback');
+
+setup(Fallback, './fixtures/astro-fallback');
+
+Fallback('Shows static content', async (context) => {
+  const result = await context.runtime.load('/');
+
+  assert.equal(result.statusCode, 200);
+
+  const $ = doc(result.contents);
+  assert.equal($('#fallback').text(), 'static');
+});
+
+Fallback.run();