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) { // Specifies what to do if the Future completes successfully.
+        print(content); // Executes when the file has been successfully read.
+      })
+      .catchError((error) { // Specifies error handling code.
+        print("Sorry, no news today. Here's why:\n");
+        print('$error');
+      });
+}
+
+main() {
+  printDailyNewsDigest();
+}