Turn compile-time errors into static warnings #1031
Description
There are a number of situations that are specified to be compile-time errors, but which might be treated as static warnings instead. This would be desirable, because compile-time errors impose a workflow: one must resolve them before moving forward to testing/running the program. It would be more in keeping with the spirit of Dart to "get out of the developer's way".
We should do a systematic sweep of these issues and decide which ones can be liberalized in this fashion.