Make build incremental

runtime/bin/bin.gypi

Index: runtime/bin/bin.gypi
diff --git a/runtime/bin/bin.gypi b/runtime/bin/bin.gypi
index 3637a4f40ef0da4b4266c148da990efe089dfd39..dfe9a74698b3954c2c1bd1c5fdf22eaf0488e9c0 100644
--- a/runtime/bin/bin.gypi
+++ b/runtime/bin/bin.gypi
@@ -13,6 +13,7 @@
     'snapshot_in_cc_file': 'snapshot_in.cc',
     'vm_isolate_snapshot_bin_file': '<(gen_source_dir)/vm_isolate_snapshot_gen.bin',
     'isolate_snapshot_bin_file': '<(gen_source_dir)/isolate_snapshot_gen.bin',
+    'gen_snapshot_stamp_file': '<(gen_source_dir)/gen_snapshot.stamp',
     'resources_cc_file': '<(gen_source_dir)/resources_gen.cc',
     'bootstrap_resources_cc_file':
         '<(gen_source_dir)/bootstrap_resources_gen.cc',
@@ -420,8 +421,7 @@
             '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)gen_snapshot<(EXECUTABLE_SUFFIX)',
           ],
           'outputs': [
-            '<(vm_isolate_snapshot_bin_file)',
-            '<(isolate_snapshot_bin_file)',
+            '<(gen_snapshot_stamp_file)',
           ],
           'action': [
             'python',
@@ -430,7 +430,8 @@
             '<(PRODUCT_DIR)/<(EXECUTABLE_PREFIX)gen_snapshot<(EXECUTABLE_SUFFIX)',
             '--vm_output_bin', '<(vm_isolate_snapshot_bin_file)',
             '--output_bin', '<(isolate_snapshot_bin_file)',
-            '--target_os', '<(OS)'
+            '--target_os', '<(OS)',
+            '--timestamp_file', '<(gen_snapshot_stamp_file)',
           ],
           'message': 'Generating ''<(vm_isolate_snapshot_bin_file)'' ''<(isolate_snapshot_bin_file)'' files.'
         },
@@ -449,9 +450,8 @@
           'action_name': 'generate_snapshot_file',
           'inputs': [
             '../tools/create_snapshot_file.py',
+            '<(gen_snapshot_stamp_file)',
             '<(snapshot_in_cc_file)',
-            '<(vm_isolate_snapshot_bin_file)',
-            '<(isolate_snapshot_bin_file)',
           ],
           'outputs': [
             '<(snapshot_cc_file)',