Add “pub cache repair” to forcibly re-install previously cached packages.

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

Index: sdk/lib/_internal/pub/lib/src/source_registry.dart
diff --git a/sdk/lib/_internal/pub/lib/src/source_registry.dart b/sdk/lib/_internal/pub/lib/src/source_registry.dart
index cd3dc3ef162cbd46ccfcd330ded435c71a505897..9d05e153274ccfd632fe851e510e0f5c0aef3a09 100644
--- a/sdk/lib/_internal/pub/lib/src/source_registry.dart
+++ b/sdk/lib/_internal/pub/lib/src/source_registry.dart
@@ -4,10 +4,12 @@
 
 library pub.source_registry;
 
+import 'dart:collection';
+
 import 'source.dart';
 
 /// A class that keeps track of [Source]s used for getting packages.
-class SourceRegistry {
+class SourceRegistry extends IterableBase<Source> {
   final Map<String, Source> _map;
   Source _default;
 
@@ -17,6 +19,13 @@ class SourceRegistry {
   /// Returns the default source, which is used when no source is specified.
   Source get defaultSource => _default;
 
+  /// Iterates over the registered sources in name order.
+  Iterator<Source> get iterator {
+    var sources = _map.values.toList();
+    sources.sort((a, b) => a.name.compareTo(b.name));
+    return sources.iterator;
+  }
+
   /// Sets the default source. This takes a string, which must be the name of a
   /// registered source.
   void setDefault(String name) {