Add validation infrastructure for "pub lish".

utils/pub/system_cache.dart

Index: utils/pub/system_cache.dart
diff --git a/utils/pub/system_cache.dart b/utils/pub/system_cache.dart
index 0e4822c6c37f00eda1a31c65ff7a59cc5e36d4ee..932cbbb48771c38daaad145a962a7a7e10d191c8 100644
--- a/utils/pub/system_cache.dart
+++ b/utils/pub/system_cache.dart
@@ -6,9 +6,12 @@ library system_cache;
 
 import 'dart:io';
 
+import 'git_source.dart';
+import 'hosted_source.dart';
 import 'io.dart';
 import 'io.dart' as io show createTempDir;
 import 'package.dart';
+import 'sdk_source.dart';
 import 'source.dart';
 import 'source_registry.dart';
 import 'utils.dart';
@@ -46,6 +49,16 @@ class SystemCache {
   : _pendingInstalls = new Map<PackageId, Future<Package>>(),
     sources = new SourceRegistry();
 
+  /// Creates a system cache and registers the standard set of sources.
+  factory SystemCache.withSources(String rootDir, String sdkDir) {
+    var cache = new SystemCache(rootDir);
+    cache.register(new SdkSource(sdkDir));
+    cache.register(new GitSource());
+    cache.register(new HostedSource());
+    cache.sources.setDefault('hosted');
+    return cache;
+  }
+
   /**
    * Registers a new source. This source must not have the same name as a source
    * that's already been registered.