Rather than ask users to pass the complicated and long string
`--extra-front-end-options=--dartdevc-canary,--dartdevc-module-format=ddc`
we want a simpler flag to enable the new DDC module system/hot reload.
Technically this flag enables the new module system, not necessarily hot
reload directly. But we only expect people to use the flag to enable hot
reload so I've chosen the name based on that.
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Co-authored-by: Nate Biggs <natebiggs@google.com>
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