Fixes bug where we would occasionally materialize a corrupted object.

runtime/vm/il_printer.cc

Index: runtime/vm/il_printer.cc
diff --git a/runtime/vm/il_printer.cc b/runtime/vm/il_printer.cc
index 38f9c2a0c6feaf2238758982bb22e89f7d46722c..db67698ecc3adffcf2e456c8bf7ea265c291f9d0 100644
--- a/runtime/vm/il_printer.cc
+++ b/runtime/vm/il_printer.cc
@@ -1057,4 +1057,11 @@ void Environment::PrintTo(BufferFormatter* f) const {
   if (outer_ != NULL) outer_->PrintTo(f);
 }
 
+const char* Environment::ToCString() const {
+  char buffer[1024];
+  BufferFormatter bf(buffer, 1024);
+  PrintTo(&bf);
+  return Isolate::Current()->current_zone()->MakeCopyOfString(buffer);
+}
+
 }  // namespace dart