use proper notation for true, false, and null in doc comments

sdk/lib/core/expando.dart

Index: sdk/lib/core/expando.dart
diff --git a/sdk/lib/core/expando.dart b/sdk/lib/core/expando.dart
index 5df0a7be47ecaaa691e4ee38440539533ae04c55..5b239de447936723e16bd7973b40330a90ca57b6 100644
--- a/sdk/lib/core/expando.dart
+++ b/sdk/lib/core/expando.dart
@@ -11,7 +11,7 @@ class Expando<T> {
 
   /**
    * The name of the this [Expando] as passed to the constructor. If
-   * no name was passed to the constructor, the name is [null].
+   * no name was passed to the constructor, the name is [:null:].
    */
   final String name;
 
@@ -31,7 +31,7 @@ class Expando<T> {
   /**
    * Gets the value of this [Expando]'s property on the given
    * object. If the object hasn't been expanded, the method returns
-   * [null].
+   * [:null:].
    */
   external T operator [](Object object);