[infra] Translate _sources.gypi files to _sources.gni files

sdk/lib/math/math_sources.gni

Index: sdk/lib/math/math_sources.gni
diff --git a/sdk/lib/math/math_sources.gni b/sdk/lib/math/math_sources.gni
new file mode 100644
index 0000000000000000000000000000000000000000..ff8f3fad45ba24f8ab77023c27c74b78472fad66
--- /dev/null
+++ b/sdk/lib/math/math_sources.gni
@@ -0,0 +1,13 @@
+# Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
+# for details. All rights reserved. Use of this source code is governed by a
+# BSD-style license that can be found in the LICENSE file.
+
+math_sdk_sources = [
+  "math.dart",
+
+  # The above file needs to be first as it lists the parts below.
+  "jenkins_smi_hash.dart",
+  "point.dart",
+  "random.dart",
+  "rectangle.dart",
+]