Refactor away from deprecated error (dart-lang/pub_semver#72)

* Refactor away from deprecated error

`FallThroughError` is deprecated, and scheduled for removal in 3.0

* Update version_constraint.dart

pkgs/pub_semver/lib/src/version_constraint.dart

diff --git a/pkgs/pub_semver/lib/src/version_constraint.dart b/pkgs/pub_semver/lib/src/version_constraint.dart
index 6eb6c3a..50d7c77 100644
--- a/pkgs/pub_semver/lib/src/version_constraint.dart
+++ b/pkgs/pub_semver/lib/src/version_constraint.dart
@@ -93,7 +93,7 @@
         case '>':
           return VersionRange(min: version, includeMin: false);
       }
-      throw FallThroughError();
+      throw UnsupportedError(op!);
     }
 
     // Try to parse the "^" operator followed by a version.