Dart schema compiler generated a static getter for enum values, which
always created a new map instance for its callers. See #5819.
Now it generates const map for better performance
and readability.
This is because they are incompatible with C++ and possibly other
languages that make them minimum size 1 (to make sure multiple
such objects don't reside at the same address). Forcing them to size
1 was also not practical, as that is requires updating the logic
of a lot of implementations and thus possibly backwards incompatible.
More here: https://github.com/google/flatbuffers/issues/4122
Change-Id: I2bfdc8597b7cfd2235bb4074bb2ae06f81f8e57d