Simplify AST by extending LetNode and replace CommaNode.

runtime/vm/ast.cc

Index: runtime/vm/ast.cc
===================================================================
--- runtime/vm/ast.cc	(revision 23667)
+++ runtime/vm/ast.cc	(working copy)
@@ -77,7 +77,7 @@
   : AstNode(token_pos),
     vars_(1),
     initializers_(1),
-    body_(NULL) { }
+    nodes_(1) { }
 
 
 LocalVariable* LetNode::AddInitializer(AstNode* node) {
@@ -97,7 +97,9 @@
   for (intptr_t i = 0; i < num_temps(); ++i) {
     initializers_[i]->Visit(visitor);
   }
-  body_->Visit(visitor);
+  for (intptr_t i = 0; i < nodes_.length(); ++i) {
+    nodes_[i]->Visit(visitor);
+  }
 }