In Flutter, Dependencies must specify version number?

as per https://www.dartlang.org/tools/pub/dependencies

Based on what data you want to provide, you can specify dependencies in two ways. The shortest way is to just specify a name:

dependencies:
  transmogrify:

This creates a dependency on transmogrify that allows any version, and looks it up using the default source, which is pub.dartlang.org. To limit the dependency to a range of versions, you can provide a version constraint:

dependencies: transmogrify: ^1.0.0

This creates a dependency on transmogrify using the default source and allowing any version from 1.0.0 to 2.0.0 (but not including 2.0.0). See Version constraints and Caret syntax for details on the version constraint syntax.

I guess that the real answer to my question is that usually, it's best to specify a major version number ratio e.g.: ^1.0.0 == 1.0.0 < 2.0.0. This is to say that this program works and is tested and will keep working with this library dependancy so long as there are no major changes.

You can use any

dependencies:
  camera: any

Having tighter constraints makes it easier for packages get/packages upgrade to search matching versions because it reduces the solution space, but for simple examples it usually doesn't matter.

pub got an improved solver recently that makes any much less of a problem than it used to be where pub often just timed out when any was used.