Initial --modules=es6 support

lib/src/js/nodes.dart

Index: lib/src/js/nodes.dart
diff --git a/lib/src/js/nodes.dart b/lib/src/js/nodes.dart
index af0fc120c6717ec0d0d75ec84a3b48f702075534..4b3f1942f8c0fd6599bd8def0ef523596a95f061 100644
--- a/lib/src/js/nodes.dart
+++ b/lib/src/js/nodes.dart
@@ -284,13 +284,13 @@ class Program extends Node {
   final String scriptTag;
 
   /// Top-level statements in the program.
-  final List<Statement> body;
+  final List<ModuleItem> body;
 
   Program(this.body, {this.scriptTag});
 
   accept(NodeVisitor visitor) => visitor.visitProgram(this);
   void visitChildren(NodeVisitor visitor) {
-    for (Statement statement in body) statement.accept(visitor);
+    for (ModuleItem statement in body) statement.accept(visitor);
   }
   Program _clone() => new Program(body);
 }