[gn] Consolidate exec_script calls to speed up generation

runtime/bin/gypi_contents.gni

Index: runtime/bin/gypi_contents.gni
diff --git a/runtime/bin/gypi_contents.gni b/runtime/bin/gypi_contents.gni
new file mode 100644
index 0000000000000000000000000000000000000000..34fc2692964e448ee94fd19e726ac869670ce8c9
--- /dev/null
+++ b/runtime/bin/gypi_contents.gni
@@ -0,0 +1,43 @@
+# Copyright (c) 2016, 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.
+
+_gypi_files = [
+  "builtin_sources.gypi",
+  "builtin_impl_sources.gypi",
+  "vmservice/vmservice_sources.gypi",
+  "io_sources.gypi",
+  "io_impl_sources.gypi",
+]
+
+_gypi_contents = exec_script("../../tools/gypi_to_gn.py",
+                             rebase_path(_gypi_files) + [ "--prefix" ],
+                             "scope",
+                             _gypi_files)
+
+resources_sources_gypi = _gypi_contents.vmservice_sources_sources
+
+builtin_sources_gypi = _gypi_contents.builtin_sources_sources
+builtin_impl_sources_gypi = _gypi_contents.builtin_impl_sources_sources
+
+io_sources_gypi = _gypi_contents.io_sources_sources
+io_impl_sources_gypi = _gypi_contents.io_impl_sources_sources
+
+_test_gypi_files = [
+  "../vm/vm_sources.gypi",
+  "builtin_impl_sources.gypi",
+]
+
+_test_only_gypi_contents = exec_script("../../tools/gypi_to_gn.py",
+                                       rebase_path(_test_gypi_files) + [
+                                             "--keep_only=_test.cc",
+                                             "--keep_only=_test.h",
+                                             "--prefix",
+                                           ],
+                                       "scope",
+                                       _test_gypi_files)
+
+# The VM sources are already included in libdart, so we just want to add in
+# the tests here.
+vm_tests_list = _test_only_gypi_contents.vm_sources_sources
+builtin_impl_tests_list = _test_only_gypi_contents.builtin_impl_sources_sources