Refactor Source and SourceRegistry.

lib/src/source/unknown.dart

Index: lib/src/source/unknown.dart
diff --git a/lib/src/source/unknown.dart b/lib/src/source/unknown.dart
index 7f015af00f8847417150238b48e2b3f7d894bf00..f4107e6e21c2b163a238801de90dcc66423df561 100644
--- a/lib/src/source/unknown.dart
+++ b/lib/src/source/unknown.dart
@@ -9,6 +9,7 @@ import 'package:pub_semver/pub_semver.dart';
 import '../package.dart';
 import '../pubspec.dart';
 import '../source.dart';
+import '../system_cache.dart';
 
 /// A [Null Object] that represents a source not recognized by pub.
 ///
@@ -21,6 +22,9 @@ class UnknownSource extends Source {
 
   UnknownSource(this.name);
 
+  LiveSource bind(SystemCache systemCache) =>
+      new _LiveUnknownSource(this, systemCache);
+
   /// Two unknown sources are the same if their names are the same.
   bool operator==(other) =>
       other is UnknownSource &&
@@ -28,26 +32,34 @@ class UnknownSource extends Source {
 
   int get hashCode => name.hashCode;
 
+  bool descriptionsEqual(description1, description2) =>
+      description1 == description2;
+
+  PackageRef parseRef(String name, description, {String containingPath}) =>
+      new PackageRef(name, this.name, description);
+
+  PackageId parseId(String name, Version version, description) =>
+      new PackageId(name, this.name, version, description);
+}
+
+class _LiveUnknownSource extends LiveSource {
+  final UnknownSource source;
+
+  final SystemCache systemCache;
+
+  _LiveUnknownSource(this.source, this.systemCache);
+
   Future<List<PackageId>> doGetVersions(PackageRef ref) =>
       throw new UnsupportedError(
-          "Cannot get package versions from unknown source '$name'.");
+          "Cannot get package versions from unknown source '${source.name}'.");
 
   Future<Pubspec> doDescribe(PackageId id) => throw new UnsupportedError(
-      "Cannot describe a package from unknown source '$name'.");
+      "Cannot describe a package from unknown source '${source.name}'.");
 
   Future get(PackageId id, String symlink) => throw new UnsupportedError(
-      "Cannot get an unknown source '$name'.");
+      "Cannot get an unknown source '${source.name}'.");
 
   /// Returns the directory where this package can be found locally.
   String getDirectory(PackageId id) => throw new UnsupportedError(
-      "Cannot find a package from an unknown source '$name'.");
-
-  bool descriptionsEqual(description1, description2) =>
-      description1 == description2;
-
-  PackageRef parseRef(String name, description, {String containingPath}) =>
-      new PackageRef(name, this.name, description);
-
-  PackageId parseId(String name, Version version, description) =>
-      new PackageId(name, this.name, version, description);
+      "Cannot find a package from an unknown source '${source.name}'.");
 }