Add validation infrastructure for "pub lish".

utils/pub/validator.dart

Index: utils/pub/validator.dart
diff --git a/utils/pub/validator.dart b/utils/pub/validator.dart
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+++ b/utils/pub/validator.dart
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+// 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 validator;
+
+import 'entrypoint.dart';
+import 'io.dart';
+import 'system_cache.dart';
+import 'utils.dart';
+import 'validator/pubspec_field.dart';
+
+/// The base class for validators that check whether a package is fit for
+/// uploading. Each validator should override [errors], [warnings], or both to
+/// return lists of errors or warnings to display to the user. Errors will cause
+/// the package not to be uploaded; warnings will require the user to confirm
+/// the upload.
+abstract class Validator {
+  /// The entrypoint that's being validated.
+  final Entrypoint entrypoint;
+
+  /// The accumulated errors for this validator. Filled by calling [validate].
+  final errors = <String>[];
+
+  /// The accumulated warnings for this validator. Filled by calling [validate].
+  final warnings = <String>[];
+
+  Validator(this.entrypoint);
+
+  /// Validates the entrypoint, adding any errors and warnings to [errors] and
+  /// [warnings], respectively.
+  Future validate();
+
+  /// Run all validators on the [entrypoint] package and print their results.
+  /// The future will complete with the error and warning messages,
+  /// respectively.
+  static Future<Pair<List<String>, List<String>>> runAll(
+      Entrypoint entrypoint) {
+    var validators = [
+      new PubspecFieldValidator(entrypoint)
+    ];
+
+    // TODO(nweiz): The sleep 0 here forces us to go async. This works around
+    // 3356, which causes a bug if all validators are (synchronously) using
+    // Future.immediate and an error is thrown before a handler is set up.
+    return sleep(0).chain((_) {
+      return Futures.wait(validators.map((validator) => validator.validate()));
+    }).transform((_) {
+      var errors = flatten(validators.map((validator) => validator.errors));
+      var warnings = flatten(validators.map((validator) => validator.warnings));
+
+      if (!errors.isEmpty) {
+        printError("== Errors:");
+        for (var error in errors) {
+          printError("* $error");
+        }
+        printError("");
+      }
+
+      if (!warnings.isEmpty) {
+        printError("== Warnings:");
+        for (var warning in warnings) {
+          printError("* $warning");
+        }
+        printError("");
+      }
+
+      return new Pair<List<String>, List<String>>(errors, warnings);
+    });
+  }
+}