Lazily format LanguageError messages (fix issue 15069).

runtime/vm/flow_graph.cc

Index: runtime/vm/flow_graph.cc
===================================================================
--- runtime/vm/flow_graph.cc	(revision 30304)
+++ runtime/vm/flow_graph.cc	(working copy)
@@ -1067,13 +1067,16 @@
 
 
 void FlowGraph::Bailout(const char* reason) const {
-  const char* kFormat = "FlowGraph Bailout: %s %s";
-  const char* function_name = parsed_function_.function().ToCString();
-  intptr_t len = OS::SNPrint(NULL, 0, kFormat, function_name, reason) + 1;
-  char* chars = Isolate::Current()->current_zone()->Alloc<char>(len);
-  OS::SNPrint(chars, len, kFormat, function_name, reason);
+  const Function& function = parsed_function_.function();
   const Error& error = Error::Handle(
-      LanguageError::New(String::Handle(String::New(chars))));
+      LanguageError::NewFormatted(Error::Handle(),  // No previous error.
+                                  Script::Handle(function.script()),
+                                  function.token_pos(),
+                                  LanguageError::kError,
+                                  Heap::kNew,
+                                  "FlowGraph Bailout: %s %s",
+                                  String::Handle(function.name()).ToCString(),
+                                  reason));
   Isolate::Current()->long_jump_base()->Jump(1, error);
 }