Add validation infrastructure for "pub lish".

utils/pub/pubspec.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 pubspec;
 
 import 'package.dart';
 import 'source.dart';
 import 'source_registry.dart';
 import 'utils.dart';
@@ skipping 10 matching lines @@
   final String name;
 
   /**
    * This package's version.
    */
   final Version version;
 
   /**
    * The packages this package depends on.
    */
-  List<PackageRef> dependencies;
+  final List<PackageRef> dependencies;
 
-  Pubspec(this.name, this.version, this.dependencies);
+  /// All pubspec fields. This includes the fields from which other properties
+  /// are derived.
+  final Map<String, Object> fields;
+
+  Pubspec(this.name, this.version, this.dependencies,
+      [Map<String, Object> fields])
+    : this.fields = fields == null ? {} : fields;
 
   Pubspec.empty()
     : name = null,
       version = Version.none,
-      dependencies = <PackageRef>[];
+      dependencies = <PackageRef>[],
+      fields = {};
 
   /** Whether or not the pubspec has no contents. */
   bool get isEmpty =>
     name == null && version == Version.none && dependencies.isEmpty;
 
   /**
    * Parses the pubspec whose text is [contents]. If the pubspec doesn't define
    * version for itself, it defaults to [Version.none].
    */
   factory Pubspec.parse(String contents, SourceRegistry sources) {
@@ skipping 70 matching lines @@
         throw new FormatException('The pubspec "authors" field should be a '
             'string or a list of strings, but was "$authors".');
       }
 
       if (parsedPubspec.containsKey('author')) {
         throw new FormatException('A pubspec should not have both an "author" '
             'and an "authors" field.');
       }
     }
 
-    return new Pubspec(name, version, dependencies);
+    return new Pubspec(name, version, dependencies, parsedPubspec);
   }
 }
 
 List<PackageRef> _parseDependencies(SourceRegistry sources, yaml) {
   var dependencies = <PackageRef>[];
 
   // Allow an empty dependencies key.
   if (yaml == null) return dependencies;
 
   if (yaml is! Map || yaml.keys.some((e) => e is! String)) {
@@ skipping 37 matching lines @@
     }
 
     source.validateDescription(description, fromLockFile: false);
 
     dependencies.add(new PackageRef(
         name, source, versionConstraint, description));
   });
 
   return dependencies;
 }