Identification for synthetic kernel nodes

[nshahan]

@johnniwinther

Thanks for a helpful discussion about the synthetic fields that appear from the late field lowering. As we discussed I'm opening this issue to help track adding flags for all of the synthetic nodes that are created from the use of late beyond just class fields.

During the discussion some idea that came up include

• A flag to identify synthetic nodes.
• An API to map synthetic names to the actual name in the dart source file.

Using the existing names generated by the CFE we have a workaround that we can use for the short term so this does not have urgent priority but it will be helpful for our future efforts and to make our implementation more resilient to changes in the naming that the CFE uses.

[johnniwinther]

With https://dart-review.googlesource.com/c/sdk/+/175250 local variable now have a isLowered flag and functions have been added to lowering_predicates.dart to extract the original name for lowered variables.