Add offline support to pub install and update.

sdk/lib/_internal/pub/lib/src/system_cache.dart

Index: sdk/lib/_internal/pub/lib/src/system_cache.dart
diff --git a/sdk/lib/_internal/pub/lib/src/system_cache.dart b/sdk/lib/_internal/pub/lib/src/system_cache.dart
index 8a51912a6eb40de1247d4f5d47a647ee98579144..567803bbe46e95f3c544d8b4ca0dde4eed4d0268 100644
--- a/sdk/lib/_internal/pub/lib/src/system_cache.dart
+++ b/sdk/lib/_internal/pub/lib/src/system_cache.dart
@@ -45,11 +45,19 @@ 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) {
+  /// Creates a system cache and registers the standard set of sources. If
+  /// [isOffline] is `true`, then the offline hosted source will be used.
+  /// Defaults to `false`.
+  factory SystemCache.withSources(String rootDir, {bool isOffline: false}) {
     var cache = new SystemCache(rootDir);
     cache.register(new GitSource());
-    cache.register(new HostedSource());
+
+    if (isOffline) {
+      cache.register(new OfflineHostedSource());
+    } else {
+      cache.register(new HostedSource());
+    }
+
     cache.register(new PathSource());
     cache.sources.setDefault('hosted');
     return cache;