Gracefully handle pubspecs with dependencies using unknown sources.

sdk/lib/_internal/pub/test/test_pub.dart

Index: sdk/lib/_internal/pub/test/test_pub.dart
diff --git a/sdk/lib/_internal/pub/test/test_pub.dart b/sdk/lib/_internal/pub/test/test_pub.dart
index 8d417edd1ef6aa220a942e84840e56cbcb4c9c30..37bf6d0b452120365b208c03c6eebbf1cb734518 100644
--- a/sdk/lib/_internal/pub/test/test_pub.dart
+++ b/sdk/lib/_internal/pub/test/test_pub.dart
@@ -545,20 +545,6 @@ Future<Map> dependencyListToMap(List<Map> dependencies) {
     for (var dependency in resolvedDependencies) {
       var keys = dependency.keys.where((key) => key != "version");
       var sourceName = only(keys);
-      var source;
-      switch (sourceName) {
-      case "git":
-        source = new GitSource();
-        break;
-      case "hosted":
-        source = new HostedSource();
-        break;
-      case "path":
-        source = new PathSource();
-        break;
-      default:
-        throw new Exception('Unknown source "$sourceName"');
-      }
 
       result[_packageName(sourceName, dependency[sourceName])] = dependency;
     }