Improve the heuristics for "pub get" is needed.

lib/src/command/deps.dart

 // Copyright (c) 2014, 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.
 
 library pub.command.list;
 
 import 'dart:collection';
 
 import '../ascii_tree.dart' as tree;
 import '../command.dart';
@@ skipping 56 matching lines @@
     _outputCompactPackages("transitive dependencies", transitive);
   }
 
   /// Outputs one section of packages in the compact output.
   _outputCompactPackages(String section, Iterable<String> names) {
     if (names.isEmpty) return;
 
     _buffer.writeln();
     _buffer.writeln("$section:");
     for (var name in ordered(names)) {
-      var package = entrypoint.packageGraph.packages[name];
+      var package = _getPackage(name);
 
       _buffer.write("- ${_labelPackage(package)}");
       if (package.dependencies.isEmpty) {
         _buffer.writeln();
       } else {
         var depNames = package.dependencies.map((dep) => dep.name);
         var depsList = "[${depNames.join(' ')}]";
         _buffer.writeln(" ${log.gray(depsList)}");
       }
     }
@@ skipping 20 matching lines @@
   }
 
   /// Outputs one section of packages in the list output.
   _outputListSection(String name, Iterable<String> deps) {
     if (deps.isEmpty) return;
 
     _buffer.writeln();
     _buffer.writeln("$name:");
 
     for (var name in deps) {
-      var package = entrypoint.packageGraph.packages[name];
+      var package = _getPackage(name);
       _buffer.writeln("- ${_labelPackage(package)}");
 
       for (var dep in package.dependencies) {
         _buffer.writeln(
             "  - ${log.bold(dep.name)} ${log.gray(dep.constraint)}");
       }
     }
   }
 
   /// Generates a dependency tree for the root package.
   ///
   /// If a package is encountered more than once (i.e. a shared or circular
   /// dependency), later ones are not traversed. This is done in breadth-first
   /// fashion so that a package will always be expanded at the shallowest
   /// depth that it appears at.
   void _outputTree() {
     // The work list for the breadth-first traversal. It contains the package
     // being added to the tree, and the parent map that will receive that
     // package.
     var toWalk = new Queue<Pair<Package, Map>>();
     var visited = new Set<String>.from([entrypoint.root.name]);
 
     // Start with the root dependencies.
     var packageTree = {};
     for (var dep in entrypoint.root.immediateDependencies) {
-      toWalk.add(
-          new Pair(entrypoint.packageGraph.packages[dep.name], packageTree));
+      toWalk.add(new Pair(_getPackage(dep.name), packageTree));
     }
 
     // Do a breadth-first walk to the dependency graph.
     while (toWalk.isNotEmpty) {
       var pair = toWalk.removeFirst();
       var package = pair.first;
       var map = pair.last;
 
       if (visited.contains(package.name)) {
         map[log.gray('${package.name}...')] = {};
         continue;
       }
 
       visited.add(package.name);
 
       // Populate the map with this package's dependencies.
       var childMap = {};
       map[_labelPackage(package)] = childMap;
 
       for (var dep in package.dependencies) {
-        toWalk.add(
-            new Pair(entrypoint.packageGraph.packages[dep.name], childMap));
+        toWalk.add(new Pair(_getPackage(dep.name), childMap));
       }
     }
 
     _buffer.write(tree.fromMap(packageTree, showAllChildren: true));
   }
 
   String _labelPackage(Package package) =>
       "${log.bold(package.name)} ${package.version}";
 
   /// Gets the names of the non-immediate dependencies of the root package.
   Set<String> _getTransitiveDependencies() {
     var transitive = entrypoint.packageGraph.packages.keys.toSet();
     var root = entrypoint.root;
     transitive.remove(root.name);
     transitive.removeAll(root.dependencies.map((dep) => dep.name));
     transitive.removeAll(root.devDependencies.map((dep) => dep.name));
     transitive.removeAll(root.dependencyOverrides.map((dep) => dep.name));
     return transitive;
   }
+
+  /// Get the package named [name], or throw a [DataError] if it's not
+  /// available.
+  ///
+  /// It's very unlikely that the lockfile won't be up-to-date with the pubspec,
+  /// but it's possible, since [Entrypoint.assertUpToDate]'s modification time
+  /// check can return a false negative. This fails gracefully if that happens.
+  Package _getPackage(String name) {
+    var package = entrypoint.packageGraph.packages[name];
+    if (package != null) return package;
+    dataError('The pubspec.yaml file has changed since the pubspec.lock file '
+        'was generated, please run "pub get" again.');
+  }
 }