Make mappedBy lazy.

utils/pub/version.dart

Index: utils/pub/version.dart
diff --git a/utils/pub/version.dart b/utils/pub/version.dart
index 67e1be66a22126a24d03f527ac0df290e0c5b477..072c97dbe5165af4d1a3c5887a7eb918304e76b9 100644
--- a/utils/pub/version.dart
+++ b/utils/pub/version.dart
@@ -187,7 +187,7 @@ class Version implements Comparable, VersionConstraint {
         // Not a number.
         return part;
       }
-    });
+    }).toList();
   }
 }
 
@@ -226,7 +226,7 @@ interface VersionConstraint default _VersionConstraintFactory {
    * constraints is empty, then it returns a VersionConstraint that allows all
    * versions.
    */
-  VersionConstraint.intersect(Collection<VersionConstraint> constraints);
+  VersionConstraint.intersect(Iterable<VersionConstraint> constraints);
 
   /**
    * Returns `true` if this constraint allows no versions.
@@ -393,7 +393,7 @@ class _VersionConstraintFactory {
   }
 
   factory VersionConstraint.intersect(
-      Collection<VersionConstraint> constraints) {
+      Iterable<VersionConstraint> constraints) {
     var constraint = new VersionRange();
     for (var other in constraints) {
       constraint = constraint.intersect(other);