The call to String::Flatten can cause garbage collection and in general adds
complexity to the code generation. It also blocks the way to run code generation on worker threads.

The call to String::Flatten in Constant::ToHeapObject() seems not to be necessary
for correctness. If removing this call affects performance negatively, we can revert
this CL.

Review-Url: https://codereview.chromium.org/2107243002
Cr-Commit-Position: refs/heads/master@{#37422}