Validate packages against their SDK constraints.

utils/pub/package.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.
 
 library package;
 
 import 'dart:async';
 import 'io.dart';
 import 'pubspec.dart';
 import 'source.dart';
@@ skipping 38 matching lines @@
   }
 
   /// The package's version.
   Version get version => pubspec.version;
 
   /// The parsed pubspec associated with this package.
   final Pubspec pubspec;
 
   /// The ids of the packages that this package depends on. This is what is
   /// specified in the pubspec when this package depends on another.
-  Collection<PackageRef> get dependencies => pubspec.dependencies;
+  List<PackageRef> get dependencies => pubspec.dependencies;
 
   /// Constructs a package with the given pubspec. The package will have no
   /// directory associated with it.
   Package.inMemory(this.pubspec)
     : dir = null;
 
   /// Constructs a package. This should not be called directly. Instead, acquire
   /// packages from [load()].
   Package._(this.dir, this.pubspec);
 
@@ skipping 127 matching lines @@
 
 class PubspecNameMismatchException implements Exception {
   final String expectedName;
   final String actualName;
 
   PubspecNameMismatchException(this.expectedName, this.actualName);
 
   String toString() => 'The name you specified for your dependency, '
     '"$expectedName", doesn\'t match the name "$actualName" in its pubspec.';
 }