Add a few more known non-null cases

lib/runtime/dart/_debugger.js

 dart_library.library('dart/_debugger', null, /* Imports */[
   'dart/_runtime',
   'dart/core'
 ], /* Lazy imports */[
 ], function(exports, dart, core) {
   'use strict';
   let dartx = dart.dartx;
   const skipDartConfig = dart.const(new core.Object());
   exports.maxIterableChildrenToDisplay = 50;
   dart.defineLazyProperties(exports, {
@@ skipping 179 matching lines @@
       addAttribute: [dart.dynamic, [dart.dynamic, dart.dynamic]],
       createTextChild: [dart.dynamic, [core.String]],
       toJsonML: [dart.dynamic, []]
     })
   });
   class JsonMLFormatter extends core.Object {
     JsonMLFormatter(simpleFormatter) {
       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');
       element.setStyle('background-color: #d9edf7');
       element.createTextChild(c);
       return element.toJsonML();
     }
     hasBody(object) {
       return this[_simpleFormatter].hasChildren(object);
     }
@@ skipping 345 matching lines @@
   exports.DartFormatter = DartFormatter;
   exports.ObjectFormatter = ObjectFormatter;
   exports.FunctionFormatter = FunctionFormatter;
   exports.MapFormatter = MapFormatter;
   exports.IterableFormatter = IterableFormatter;
   exports.ClassMetadataFormatter = ClassMetadataFormatter;
   exports.MapEntryFormatter = MapEntryFormatter;
   exports.HeritageClauseFormatter = HeritageClauseFormatter;
   exports.registerDevtoolsFormatter = registerDevtoolsFormatter;
 });