Make mappedBy lazy.

utils/pub/version_solver.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * Attempts to resolve a set of version constraints for a package dependency
  * graph and select an appropriate set of best specific versions for all
  * dependent packages. It works iteratively and tries to reach a stable
  * solution where the constraints of all dependencies are met. If it fails to
  * reach a solution after a certain number of iterations, it assumes the
@@ skipping 201 matching lines @@
       // use that. This allows e.g. Git to ensure that the same commit is used.
       var lockedPackage = lockFile.packages[dep.name];
       if (lockedPackage != null && lockedPackage.version == dep.version &&
           lockedPackage.source.name == dep.source.name &&
           dep.source.descriptionsEqual(
               description, lockedPackage.description)) {
         description = lockedPackage.description;
       }
 
       return new PackageId(dep.name, dep.source, dep.version, description);
-    });
+    })
+    .toList();
   }
 }
 
 /**
  * The constraint solver works by iteratively processing a queue of work items.
  * Each item is a single atomic change to the dependency graph. Handling them
  * in a queue lets us handle asynchrony (resolving versions requires information
  * from servers) as well as avoid deeply nested recursion.
 */
 abstract class WorkItem {
@@ skipping 492 matching lines @@
   final description1;
   final description2;
 
   DescriptionMismatchException(this.package, this.description1,
       this.description2);
 
   // TODO(nweiz): Dump to YAML when that's supported
   String toString() => "Package '$package' has conflicting descriptions "
     "'${json.stringify(description1)}' and '${json.stringify(description2)}'";
 }