Add VersionConstraint.difference().

lib/src/version_constraint.dart

Index: lib/src/version_constraint.dart
diff --git a/lib/src/version_constraint.dart b/lib/src/version_constraint.dart
index 68827a40c4c9084336df94f3cc7bac4f973a1678..147a732d530b3629876fb1d94b9cb6033d76414a 100644
--- a/lib/src/version_constraint.dart
+++ b/lib/src/version_constraint.dart
@@ -236,6 +236,10 @@ abstract class VersionConstraint {
   /// Creates a new [VersionConstraint] that allows [Versions]s allowed by
   /// either this or [other].
   VersionConstraint union(VersionConstraint other);
+
+  /// Creates a new [VersionConstraint] that allows [Version]s allowed by this

[Bob Nystrom, 2016/06/08 22:51:06]

Technically, these don't always create *new* constraints. How about just
"Returns a [VersionConstraint]..."

[nweiz, 2016/06/08 23:39:02]

Done.

+  /// but not [other].
+  VersionConstraint difference(VersionConstraint other);
 }
 
 class _EmptyVersion implements VersionConstraint {
@@ -248,6 +252,7 @@ class _EmptyVersion implements VersionConstraint {
   bool allowsAny(VersionConstraint other) => false;
   VersionConstraint intersect(VersionConstraint other) => this;
   VersionConstraint union(VersionConstraint other) => other;
+  VersionConstraint difference(VersionConstraint other) => this;
   String toString() => '<empty>';
 }