Fuchsia: Make targets including the observatory in the standlone VM available.

runtime/observatory/BUILD.gn

Index: runtime/observatory/BUILD.gn
diff --git a/runtime/observatory/BUILD.gn b/runtime/observatory/BUILD.gn
index d4c736ca876e67179de01ab5bec8fa6797b888ec..c55b92d9143acca3fa0a99e83a841e314e1e29f7 100644
--- a/runtime/observatory/BUILD.gn
+++ b/runtime/observatory/BUILD.gn
@@ -22,6 +22,11 @@ declare_args() {
   dart_host_pub_exe = rebase_path("$dart_host_sdk/bin/pub")
 }
 
+# We set this to "" in the Fuchsia build to force building with dart_bootstrap.
+if (defined(is_fuchsia) && is_fuchsia) {
+  dart_host_pub_exe = ""
+}
+
 # Construct arguments to the observatory tool for finding pub.
 pub_build_deps = []
 pub_build_args = []