Make it possible to share the embedder's dart sources for the vmservice

runtime/bin/BUILD.gn

Index: runtime/bin/BUILD.gn
diff --git a/runtime/bin/BUILD.gn b/runtime/bin/BUILD.gn
index b6d6ac1f26cbfdb0755af841c7c76c4396e99462..b963f09aafb55c76b2d88b852d9c5967c0f73891 100644
--- a/runtime/bin/BUILD.gn
+++ b/runtime/bin/BUILD.gn
@@ -10,9 +10,9 @@ declare_args() {
 
 resources_sources_gypi =
     exec_script("../../tools/gypi_to_gn.py",
-                [rebase_path("resources_sources.gypi")],
+                [rebase_path("vmservice/vmservice_sources.gypi")],
                 "scope",
-                ["resources_sources.gypi"])
+                ["vmservice/vmservice_sources.gypi"])
 
 # Generate a resources.cc file for the service isolate without Observatory.
 action("gen_resources_cc") {
@@ -21,7 +21,9 @@ action("gen_resources_cc") {
   inputs = [
     "../tools/create_resources.py",
   ]
-  sources = resources_sources_gypi.sources
+  sources = rebase_path(resources_sources_gypi.sources,
+                        "",
+                        "//dart/runtime/bin/vmservice/")

[zra, 2016/01/28 18:40:04]

Is there any way to avoid using a path here that is specific to the Mojo repo?

[Cutch, 2016/01/28 23:49:44]

Not that I'm aware of. Have any of these BUILD.gn files been exercised outside
of mojo or flutter repositories?

[zra, 2016/01/29 02:58:18]

No, but it would one day be nice to switch the Dart build from gyp to GN. Maybe
it's a bit of a pipe dream, though. Would you mind adding a note just in case?

[Cutch, 2016/01/29 15:36:52]

I've left a note. I would be really pleased if we switched to GN.

   outputs = [ "$target_gen_dir/resources_gen.cc" ]
   args = [
     "--output",