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

runtime/vm/flow_graph_builder.cc

Index: runtime/vm/flow_graph_builder.cc
===================================================================
--- runtime/vm/flow_graph_builder.cc	(revision 28647)
+++ runtime/vm/flow_graph_builder.cc	(working copy)
@@ -2032,6 +2032,17 @@
 }
 
 
+void EffectGraphVisitor::VisitStringInterpolateNode(
+    StringInterpolateNode* node) {
+  ValueGraphVisitor for_argument(owner(), temp_index());
+  node->value()->Visit(&for_argument);
+  Append(for_argument);
+  StringInterpolateInstr* instr =
+      new StringInterpolateInstr(for_argument.value(), node->token_pos());
+  ReturnDefinition(instr);
+}
+
+
 void EffectGraphVisitor::VisitClosureNode(ClosureNode* node) {
   const Function& function = node->function();