added front page samples to testing, plus a couple of tweaks to front page and code lab

src/tests/site/code/async.dart

+import 'dart:async';
+import 'dart:html';
+
+void main() {
+  // Get a stream of key-press events from an element:
+  querySelector('textarea').onKeyPress
+
+    // Filter the stream, getting a stream of events only for certain key codes:
+    .where((e) => e.keyCode >= 32 && e.keyCode <= 122)
+
+    // Convert those key codes, getting a stream of corresponding characters:
+    .map((e) => new String.fromCharCode(e.charCode))
+
+    // Ask for the first character, getting a Future.
+    .first
+
+    // When the Future completes, print the first character to the console.
+    .then((char) => print('First char=$char'));
+}