Show better error messages in network failures.

utils/pub/source.dart

Index: utils/pub/source.dart
diff --git a/utils/pub/source.dart b/utils/pub/source.dart
index bb956c95c106916c1979ae8c89814173beb543b5..0dc6a23bca7f3de35a6aeb1967f42d68ff5bf7c6 100644
--- a/utils/pub/source.dart
+++ b/utils/pub/source.dart
@@ -23,6 +23,9 @@ abstract class Source {
    */
   abstract String get name;
 
+  /// Whether or not this source is the default source.
+  bool get isDefault => systemCache.sources.defaultSource == this;
+
   /**
    * Whether this source's packages should be cached in Pub's global cache
    * directory.
@@ -189,4 +192,7 @@ abstract class Source {
    * By default, this just returns [id].
    */
   Future<PackageId> resolveId(PackageId id) => new Future.immediate(id);
+
+  /// Returns the source's name.
+  String toString() => name;
 }