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

runtime/vm/flow_graph_type_propagator.cc

Index: runtime/vm/flow_graph_type_propagator.cc
===================================================================
--- runtime/vm/flow_graph_type_propagator.cc	(revision 28647)
+++ runtime/vm/flow_graph_type_propagator.cc	(working copy)
@@ -466,6 +466,11 @@
 }
 
 
+CompileType CompileType::String() {
+  return FromAbstractType(Type::ZoneHandle(Type::StringType()), kNonNullable);
+}
+
+
 intptr_t CompileType::ToCid() {
   if ((cid_ == kNullCid) || (cid_ == kDynamicCid)) {
     return cid_;
@@ -877,6 +882,12 @@
 }
 
 
+CompileType StringInterpolateInstr::ComputeType() const {
+  // TODO(srdjan): Do better and determine if it is a one or two byte string.
+  return CompileType::String();
+}
+
+
 CompileType* StoreInstanceFieldInstr::ComputeInitialType() const {
   return value()->Type();
 }