Add a few more known non-null cases

lib/runtime/dart/_debugger.js

Index: lib/runtime/dart/_debugger.js
diff --git a/lib/runtime/dart/_debugger.js b/lib/runtime/dart/_debugger.js
index 0f7507a442a0ac8a72cbe78cc4f437dfba66d3fb..ddaa60a5596b489a0bee855bbcf991e7ae08bdf8 100644
--- a/lib/runtime/dart/_debugger.js
+++ b/lib/runtime/dart/_debugger.js
@@ -197,7 +197,7 @@ dart_library.library('dart/_debugger', null, /* Imports */[
       this[_simpleFormatter] = simpleFormatter;
     }
     header(object, config) {
-      if (dart.notNull(core.identical(config, skipDartConfig))) return null;
+      if (core.identical(config, skipDartConfig)) return null;
       let c = this[_simpleFormatter].preview(object);
       if (c == null) return null;
       let element = new JsonMLElement('span');