Support "pub downgrade".

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

Index: sdk/lib/_internal/pub/lib/src/source.dart
diff --git a/sdk/lib/_internal/pub/lib/src/source.dart b/sdk/lib/_internal/pub/lib/src/source.dart
index 0235792e09bae5f34aae4f07eccea0e4bd17199d..f7674f52b157e39708d8c43c7a8f08ac8338df85 100644
--- a/sdk/lib/_internal/pub/lib/src/source.dart
+++ b/sdk/lib/_internal/pub/lib/src/source.dart
@@ -27,6 +27,10 @@ abstract class Source {
   /// all sources.
   String get name;
 
+  /// Whether this source can choose between multiple versions of the same
+  /// package during version solving.
+  bool get hasMultipleVersions;

[Bob Nystrom, 2014/07/14 22:10:07]

I would default this to false here so you don't have to bother overriding it
everywhere.

[nweiz, 2014/07/14 23:03:02]

Done.

+
   /// Whether or not this source is the default source.
   bool get isDefault => systemCache.sources.defaultSource == this;