Build sky/engine/tonic

This CL adds a dependency from Sky to DartVM.  We need to remove Sky from the
ASAN build because DartVM doesn't currently build in ASAN (see https://code.google.com/p/dart/issues/detail?id=22121).

R=rafaelw@chromium.org
BUG=454613

Review URL: https://codereview.chromium.org/919463004
diff --git a/BUILD.gn b/BUILD.gn
index 3044654..d23d33e 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -11,7 +11,6 @@
     "//mojo",
     "//services",
     "//shell",
-    "//sky",
     "//ui/gfx:gfx_unittests",
     "//ui/events:events_unittests",
   ]
@@ -19,6 +18,10 @@
   if (!is_android && !is_win) {
     deps += [ "//tools/xdisplaycheck" ]
   }
+
+  if (!is_asan) {
+    deps += [ "//sky" ]
+  }
 }
 
 # Deprecated name for the default build target.