Add Class.supertype.

Thanks, Michel!

doc/site/core/class.markdown

@@ -4,3 +4,23 @@
 ### **name**
 
 The name of the class.
+
+### **supertype**
+
+The superclass of this class.
+
+    :::dart
+    class Crustacean {}
+    class Crab is Crustacean {}
+
+    IO.print(Crab.supertype)        // "Crustacean"
+
+A class with no explicit superclass implicitly inherits Object:
+
+    :::dart
+    IO.print(Crustacean.supertype)  // "Object"
+
+Object forms the root of the class hierarchy and has no supertype:
+
+    :::dart
+    IO.print(Object.supertype)  // "null"