Add samples gyp target to Dart project, that builds samples/sample_extension.

samples/samples.gyp

Index: samples/samples.gyp
diff --git a/lib/math/math_sources.gypi b/samples/samples.gyp
similarity index 73%
copy from lib/math/math_sources.gypi
copy to samples/samples.gyp
index dc854da684570c574174c89c14463dd9f2901df2..571a3d70229632edfb1b824dec76cc31bc673242 100644
--- a/lib/math/math_sources.gypi
+++ b/samples/samples.gyp
@@ -3,8 +3,9 @@
 # BSD-style license that can be found in the LICENSE file.
 
 {
-  'sources': [
-    'base.dart',
-    'random.dart',
+  'includes': [
+    'sample_extension/sample_extension.gypi',
+  ],
+  'targets': [
   ],
 }