VM: Fix infinite recursion in optimization of of string interpolation.

runtime/vm/intermediate_language.cc

Index: runtime/vm/intermediate_language.cc
===================================================================
--- runtime/vm/intermediate_language.cc	(revision 33643)
+++ runtime/vm/intermediate_language.cc	(working copy)
@@ -3043,6 +3043,12 @@
   //   StoreIndexed(v2, v3, v4)   -- v3:constant index, v4: value.
   //   ..
   //   v8 <- StringInterpolate(v2)
+
+  // Don't compile-time fold when optimizing the interpolation function itself.
+  if (flow_graph->parsed_function().function().raw() == CallFunction().raw()) {
+    return this;
+  }
+
   CreateArrayInstr* create_array = value()->definition()->AsCreateArray();
   ASSERT(create_array != NULL);
   // Check if the string interpolation has only constant inputs.
@@ -3087,13 +3093,14 @@
   const Array& interpolate_arg = Array::Handle(Array::New(1));
   interpolate_arg.SetAt(0, array_argument);
   // Call interpolation function.
-  String& concatenated = String::ZoneHandle();
-  concatenated ^=
-      DartEntry::InvokeFunction(CallFunction(), interpolate_arg);
-  if (concatenated.IsUnhandledException()) {
+  const Object& result = Object::Handle(
+      DartEntry::InvokeFunction(CallFunction(), interpolate_arg));
+  if (result.IsUnhandledException()) {
     return this;
   }
-  concatenated = Symbols::New(concatenated);
+  ASSERT(result.IsString());
+  const String& concatenated =
+      String::ZoneHandle(Symbols::New(String::Cast(result)));
   return flow_graph->GetConstant(concatenated);
 }