Fixes https://github.com/flutter/flutter/issues/134279.

Changes:
* Moves all tests of `--dart-define-from-file` behavior from `build_bundle_test.dart` and `assemble_test.dart` to `flutter_command_test.dart`.
* Deletes a duplicate test of malformed JSON detection behavior.
* Renames the `useDartDefineFromFileOption` method of `FlutterCommand` to `_usesDartDefineFromFileOption`. This 1) makes the name more consistent with the other `uses*Option` methods and 2) hides the method since it is not used outside of the file.
* Renames several tests to better articulate what is under test and what the expected result is.
* Adds a test for the case where a `.env` file with a malformed line is provided to `--dart-define-from-file`.