Breaking change 61264: erroneous imports will become errors

[Jens Johansen]

Issue 61264 proposes a fix for a parser bug where erroneous imports fail to generate an error as required by the specification.
Most existing import statements already conform to the spec and will not require any changes.
In cases where an import does cause an error, it can be easily rewritten to be valid (an example is provided in the issue).

If you have any thoughts or concerns, please leave your feedback on the GitHub issue: https://github.com/dart-lang/sdk/issues/61264.