Parse harmony let declarations.

src/ast.h

Index: src/ast.h
diff --git a/src/ast.h b/src/ast.h
index a735ee130d331feac5e633860920588c88a7ce89..6cada20760c34f4ce97f7519355c06bbd5964aad 100644
--- a/src/ast.h
+++ b/src/ast.h
@@ -375,9 +375,11 @@ class Declaration: public AstNode {
       : proxy_(proxy),
         mode_(mode),
         fun_(fun) {
-    ASSERT(mode == Variable::VAR || mode == Variable::CONST);
+    ASSERT(mode == Variable::VAR ||
+           mode == Variable::CONST ||
+           mode == Variable::LET);
     // At the moment there are no "const functions"'s in JavaScript...
-    ASSERT(fun == NULL || mode == Variable::VAR);
+    ASSERT(fun == NULL || mode == Variable::VAR || mode == Variable::LET);
   }
 
   DECLARE_NODE_TYPE(Declaration)