Track async causal stack traces

runtime/observatory/lib/src/elements/css/shared.css

Index: runtime/observatory/lib/src/elements/css/shared.css
diff --git a/runtime/observatory/lib/src/elements/css/shared.css b/runtime/observatory/lib/src/elements/css/shared.css
index 556d9977b380b6dc125b1d7d725b2b4f011b83c5..3727ddf5a9786379724dd09743804cce16afa746 100644
--- a/runtime/observatory/lib/src/elements/css/shared.css
+++ b/runtime/observatory/lib/src/elements/css/shared.css
@@ -899,6 +899,9 @@ debugger-frame.shadow {
   box-shadow:  0 2px 10px 0 rgba(0, 0, 0, 0.16),
                0 2px 5px 0 rgba(0, 0, 0, 0.26);
 }
+debugger-frame.causalFrame {
+  background-color: #D7CCC8;
+}
 debugger-frame.current {
   box-shadow:  0 2px 10px 0 rgba(0, 0, 0, 0.26),
                0 2px 5px 0 rgba(0, 0, 0, 0.46);