Implement ParameterMirror.{isFinal,hasDefaultValue,defaultValue}.

sdk/lib/mirrors/mirrors.dart

Index: sdk/lib/mirrors/mirrors.dart
diff --git a/sdk/lib/mirrors/mirrors.dart b/sdk/lib/mirrors/mirrors.dart
index 796889c64c0a5a475b248da7c5795358f5499d3e..4e96d84e7462198101e4dfba7ab9eb3a0c434be0 100644
--- a/sdk/lib/mirrors/mirrors.dart
+++ b/sdk/lib/mirrors/mirrors.dart
@@ -1098,16 +1098,18 @@ abstract class ParameterMirror implements VariableMirror {
   bool get isNamed;
 
   /**
-   * Returns [:true:] if the reflectee has a default value.
+   * Returns [:true:] if the reflectee has explicitly declared a default value.
    * Otherwise returns [:false:].
    */
   bool get hasDefaultValue;
 
   /**
-   * A mirror on the default value for this parameter, if it exists.
+   * If this is a required parameter, returns [:null:]. Otherwise returns a
+   * mirror on the default value for this parameter. If no default is declared
+   * for an optional parameter, the default is [:null:] and a mirror on [:null:]
+   * is returned.
    */
-  // TODO(ahe): This should return an InstanceMirror.
-  String get defaultValue;
+  InstanceMirror get defaultValue;
 }
 
 /**