Add a few more known non-null cases

lib/runtime/dart/html_common.js

 dart_library.library('dart/html_common', null, /* Imports */[
   'dart/_runtime',
   'dart/_metadata',
   'dart/core',
   'dart/html',
   'dart/typed_data',
   'dart/_js_helper',
   'dart/_native_typed_data',
   'dart/async',
   'dart/collection',
@@ skipping 263 matching lines @@
   }
   dart.fn(convertNativeToDart_SerializedScriptValue);
   class _StructuredClone extends core.Object {
     _StructuredClone() {
       this.values = [];
       this.copies = [];
     }
     findSlot(value) {
       let length = this.values[dartx.length];
       for (let i = 0; i < dart.notNull(length); i++) {
-        if (dart.notNull(core.identical(this.values[dartx.get](i), value))) return i;
+        if (core.identical(this.values[dartx.get](i), value)) return i;
       }
       this.values[dartx.add](value);
       this.copies[dartx.add](null);
       return length;
     }
     readSlot(i) {
       return this.copies[dartx.get](i);
     }
     writeSlot(i, x) {
       this.copies[dartx.set](i, x);
@@ skipping 656 matching lines @@
   exports.isImmutableJavaScriptArray = isImmutableJavaScriptArray;
   exports.isJavaScriptPromise = isJavaScriptPromise;
   exports.convertNativePromiseToDartFuture = convertNativePromiseToDartFuture;
   exports.wrap_jso = wrap_jso;
   exports.unwrap_jso = unwrap_jso;
   exports.Device = Device;
   exports.FilteredElementList = FilteredElementList;
   exports.Lists = Lists;
   exports.NodeListWrapper = NodeListWrapper;
 });