Migrate pub away from throwing strings.

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:yaml/yaml.dart';
 import 'package:pathos/path.dart' as path;
 
 import 'io.dart';
@@ skipping 36 matching lines @@
       if (pubspec.name == null) {
         throw new PubspecHasNoNameException(name);
       }
 
       if (name != null && pubspec.name != name) {
         throw new PubspecNameMismatchException(name, pubspec.name);
       }
 
       return pubspec;
     } on FormatException catch (ex) {
-      throw 'Could not parse $pubspecPath:\n${ex.message}';
+      fail('Could not parse $pubspecPath:\n${ex.message}');
     }
   }
 
   Pubspec(this.name, this.version, this.dependencies, this.devDependencies,
           this.environment, [Map<String, Object> fields])
     : this.fields = fields == null ? {} : fields;
 
   Pubspec.empty()
     : name = null,
       version = Version.none,
@@ skipping 225 matching lines @@
 /// The environment-related metadata in the pubspec. Corresponds to the data
 /// under the "environment:" key in the pubspec.
 class PubspecEnvironment {
   /// The version constraint specifying which SDK versions this package works
   /// with.
   final VersionConstraint sdkVersion;
 
   PubspecEnvironment([VersionConstraint sdk])
       : sdkVersion = sdk != null ? sdk : VersionConstraint.any;
 }