Create classes_util API, change discover to return a list instead of a dict.

tools/telemetry/telemetry/util/classes_util.py

+# Copyright 2015 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+from telemetry.core import discover
+from telemetry.internal.util import camel_case
+
+
+def DiscoverClasses(start_dir, top_level_dir, base_class,
+                    directly_constructable=False):
+  return discover.DiscoverClasses(start_dir, top_level_dir, base_class,
+                                  directly_constructable=directly_constructable)
+
+def DiscoverClassesByClassName(
+    start_dir, top_level_dir, base_class, directly_constructable=False):
+  classes_dict = {}
+  classes = discover.DiscoverClasses(
+      start_dir, top_level_dir, base_class,
+      directly_constructable=directly_constructable)
+  for class_obj in classes:
+    classes_dict[camel_case.ToUnderscore(class_obj.__name__)] = class_obj
+  return classes_dict
+
+def MaybeGetInstanceOfClass(target, start_dir, top_level_dir, cls):
+  classes = DiscoverClassesByClassName(
+      start_dir, top_level_dir, cls, directly_constructable=True)
+  return classes[target]() if target in classes else None