Start resolving classes and default constructors.

frog/leg/tree/nodes.dart

Index: frog/leg/tree/nodes.dart
===================================================================
--- frog/leg/tree/nodes.dart	(revision 1986)
+++ frog/leg/tree/nodes.dart	(working copy)
@@ -231,12 +231,14 @@
   final Token beginToken;
   final Token endToken;
   final SourceString delimiter;
+  bool isEmpty() => nodes.isEmpty();
 
   // TODO(floitsch): second argument should be this.nodes.
   NodeList([this.beginToken, nodes, this.endToken, this.delimiter])
       : _nodes = nodes;
 
   NodeList.singleton(Node node) : this(null, new Link<Node>(node));
+  NodeList.empty() : this(null, const EmptyLink<Node>());
 
   NodeList asNodeList() => this;
 
@@ -321,7 +323,7 @@
 
   final Token forToken;
 
-  For(this.initializer, this.conditionStatement, this.update, body, 
+  For(this.initializer, this.conditionStatement, this.update, body,
       this.forToken) : super(body);
 
   For asFor() => this;