Add gyp build for sample-extension.

samples/sample_extension/sample_extension.gypi

+# Copyright (c) 2012, 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.
+
+{
+  'targets': [
+    {
+      'target_name': 'sample_extension',
+      'type': 'shared_library',
+      'dependencies': [
+        'dart',
+      ],
+      'include_dirs': [
+        '../../runtime/include',
+      ],
+      'sources': [
+        'sample_extension.cc',
+        'sample_extension_dllmain_win.cc',
+      ],
+      'defines': [
+        'DART_SHARED_LIB',
+      ],
+      'conditions': [
+        ['OS=="win"', {
+          'msvs_settings': {
+            'VCLinkerTool': {
+              'AdditionalDependencies': [ 'dart.lib' ],
+              'AdditionalLibraryDirectories': [ '<(PRODUCT_DIR)' ],
+            },
+          },
+        }],
+        ['OS=="mac"', {
+          'xcode_settings': {
+            'OTHER_LDFLAGS': [ '-undefined', 'dynamic_lookup' ],
+          },
+        }],
+      ],
+    },
+  ],
+}