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

src/tests/site/code/die.dart

+import 'dart:math' show Random;        // Import a class from a library.
+
+void main() {                          // This is where the app starts executing.
+  print(new Die(n: 12).roll());         // Print a new object's value. Chain method calls.
+}
+
+class Die {                            // Define a class.
+  static Random shaker = new Random(); // Define a class variable.
+  int sides, value;                    // Define instance variables.
+
+  String toString() => '$value';       // Define a method using shorthand syntax.
+
+  Die({int n: 6}) {                    // Define a constructor.
+    if (4 <= n && n <= 20) {
+      sides = n;
+    } else {
+      throw new ArgumentError(/* */);  // Support for errors and exceptions.
+    }
+  }
+  int roll() {                         // Define an instance method.
+    return value = shaker.nextInt(sides); // Get a random number.
+  }
+}