Add new style of string interpolation optimization: new nodes, working constant folding.

runtime/vm/ast.h

Index: runtime/vm/ast.h
===================================================================
--- runtime/vm/ast.h	(revision 28731)
+++ runtime/vm/ast.h	(working copy)
@@ -60,6 +60,7 @@
   V(TryCatch)                                                                  \
   V(Throw)                                                                     \
   V(InlinedFinally)                                                            \
+  V(StringInterpolate)                                                         \
 
 
 #define FORWARD_DECLARATION(BaseName) class BaseName##Node;
@@ -306,6 +307,26 @@
 };
 
 
+class StringInterpolateNode : public AstNode {
+ public:
+  StringInterpolateNode(intptr_t token_pos, ArrayNode* value)
+      : AstNode(token_pos), value_(value) { }
+
+  virtual void VisitChildren(AstNodeVisitor* visitor) const {
+    value_->Visit(visitor);
+  }
+
+  ArrayNode* value() const { return value_; }
+
+  DECLARE_COMMON_NODE_FUNCTIONS(StringInterpolateNode);
+
+ private:
+  ArrayNode* value_;
+
+  DISALLOW_IMPLICIT_CONSTRUCTORS(StringInterpolateNode);
+};
+
+
 class LiteralNode : public AstNode {
  public:
   LiteralNode(intptr_t token_pos, const Instance& literal)