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..edd8abd0729c4b38f167aad50f4fc33c06411d94 100644
--- a/lib/src/version_constraint.dart
+++ b/lib/src/version_constraint.dart
@@ -229,13 +229,17 @@ abstract class VersionConstraint {
   /// allows.
   bool allowsAny(VersionConstraint other);
 
-  /// Creates a new [VersionConstraint] that only allows [Version]s allowed by
-  /// both this and [other].
+  /// Returns a [VersionConstraint] that only allows [Version]s allowed by both
+  /// this and [other].
   VersionConstraint intersect(VersionConstraint other);
 
-  /// Creates a new [VersionConstraint] that allows [Versions]s allowed by
-  /// either this or [other].
+  /// Returns a [VersionConstraint] that allows [Versions]s allowed by either
+  /// this or [other].
   VersionConstraint union(VersionConstraint other);
+
+  /// Returns a [VersionConstraint] that allows [Version]s allowed by this 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>';
 }