Embed DartPad in the top part of homepage

src/tests/site/gists/homepage_jw/web/main.dart

Index: src/tests/site/gists/homepage_jw/web/main.dart
diff --git a/src/tests/site/gists/homepage_jw/web/main.dart b/src/tests/site/gists/homepage_jw/web/main.dart
new file mode 100644
index 0000000000000000000000000000000000000000..6c3ce84a3b6f30ffb81897324e00ca4c61a0a369
--- /dev/null
+++ b/src/tests/site/gists/homepage_jw/web/main.dart
@@ -0,0 +1,19 @@
+import 'dart:html' show HttpRequest;
+
+main() async {
+  // Asychronously get text to display.
+  var lines = await getLines();
+
+  // If result is non-null, print it.
+  lines?.forEach((line) => print(line));
+}
+
+// Reads a file, returning its lines.
+getLines() async {
+  var jabber = await HttpRequest.getString(
+      'https://www.dartlang.org/samples-files/jabberwocky.txt');
+  var lines = jabber.split('\n');
+  lines.retainWhere((line) =>
+      line.toLowerCase().contains('jabberwock'));
+  return lines;
+}