update examples on homepage

src/tests/site/code/futures.dart

+import 'dart:io';
+import 'dart:async';
+
+void printDailyNewsDigest() {
+  File file = new File('dailyNewsDigest.txt');
+  file.readAsString()                  // readAsString() returns a Future.
+    .then((content) => print(content)) // Handle successful completion.
+    .catchError((error) =>             // Handle failure.
+        print("Sorry, no news today. Here's why:\n$error"));
+}
+
+main() {
+  printDailyNewsDigest();
+}