update examples on homepage

src/tests/site/code/syntax.dart

+import 'dart:html';                      // Dart supports libraries.
+
+// Define functions either inside or outside of classes.
+startOrEndTest() {
+  // ...
+}
+
+class Rectangle implements Shape {  // Declaring a class...
+  final num height, width;           // ...with properties.
+  num get area => height * width;    // A property implemented with a getter.
+                                      // Also: function shorthand syntax (=>).
+  Rectangle(this.height, this.width); // Compact constructor syntax.
+}
+
+// Every app has a main() function, where execution starts.
+main() {
+  // The cascade operator (..) saves you from repetitive typing.
+  query("#button")
+    ..text = "Run test"
+    ..onClick.listen(startOrEndTest);
+}
+
+class Shape {}