First cut of mini dart:html.

tool/input_sdk/lib/html/html_common/conversions_dart2js.dart

+part of html_common;
+
+/// Converts a JavaScript object with properties into a Dart Map.
+/// Not suitable for nested objects.
+Map convertNativeToDart_Dictionary(object) {
+  if (object == null) return null;
+  var dict = {};
+  var keys = JS('JSExtendableArray', 'Object.getOwnPropertyNames(#)', object);
+  for (final key in keys) {
+    dict[key] = JS('var', '#[#]', object, key);
+  }
+  return dict;
+}
+
+/// Converts a flat Dart map into a JavaScript object with properties.
+convertDartToNative_Dictionary(Map dict, [void postCreate(dynamic)]) {
+  if (dict == null) return null;
+  var object = JS('var', '{}');
+  if (postCreate != null) {
+    postCreate(object);
+  }
+  dict.forEach((String key, value) {
+      JS('void', '#[#] = #', object, key, value);
+    });
+  return object;
+}
+
+
+/**
+ * Ensures that the input is a JavaScript Array.
+ *
+ * Creates a new JavaScript array if necessary, otherwise returns the original.
+ */
+List convertDartToNative_StringArray(List<String> input) {
+  // TODO(sra).  Implement this.
+  return input;
+}
+
+DateTime convertNativeToDart_DateTime(date) {
+  var millisSinceEpoch = JS('int', '#.getTime()', date);
+  return new DateTime.fromMillisecondsSinceEpoch(millisSinceEpoch, isUtc: true);
+}
+
+convertDartToNative_DateTime(DateTime date) {
+  return JS('', 'new Date(#)', date.millisecondsSinceEpoch);
+}
+
+convertDartToNative_PrepareForStructuredClone(value) =>
+    new _StructuredCloneDart2Js().convertDartToNative_PrepareForStructuredClone(value);
+
+convertNativeToDart_AcceptStructuredClone(object, {mustCopy: false}) =>
+    new _AcceptStructuredCloneDart2Js().convertNativeToDart_AcceptStructuredClone(object, mustCopy: mustCopy);
+
+class _StructuredCloneDart2Js extends _StructuredClone {
+  newJsMap() => JS('var', '{}');
+  putIntoMap(map, key, value) => JS('void', '#[#] = #', map, key, value);
+  newJsList(length) => JS('JSExtendableArray', 'new Array(#)', length);
+  cloneNotRequired(e) => (e is NativeByteBuffer || e is NativeTypedData);
+}
+
+class _AcceptStructuredCloneDart2Js extends _AcceptStructuredClone {
+
+  newJsList(length) => JS('JSExtendableArray', 'new Array(#)', length);
+  newDartList(length) => newJsList(length);
+  identicalInJs(a, b) => identical(a, b);
+
+  void forEachJsField(object, action) {
+      for (final key in JS('JSExtendableArray', 'Object.keys(#)', object)) {
+        action(key, JS('var', '#[#]', object, key));
+      }
+  }
+}
+
+bool isJavaScriptDate(value) => JS('bool', '# instanceof Date', value);
+bool isJavaScriptRegExp(value) => JS('bool', '# instanceof RegExp', value);
+bool isJavaScriptArray(value) => JS('bool', '# instanceof Array', value);
+bool isJavaScriptSimpleObject(value) {
+  var proto = JS('', 'Object.getPrototypeOf(#)', value);
+  return JS('bool', '# === Object.prototype', proto) ||
+      JS('bool', '# === null', proto);
+}
+bool isImmutableJavaScriptArray(value) =>
+    JS('bool', r'!!(#.immutable$list)', value);
+bool isJavaScriptPromise(value) =>
+    JS('bool', r'typeof Promise != "undefined" && # instanceof Promise', value);
+
+Future convertNativePromiseToDartFuture(promise) {
+  var completer = new Completer();
+  var then = convertDartClosureToJS((result) => completer.complete(result), 1);
+  var error = convertDartClosureToJS((result) => completer.completeError(result), 1);
+  var newPromise = JS('', '#.then(#).catch(#)', promise, then, error);
+  return completer.future;
+}
+
+/// Wrap a JS object with an instance of the matching dart:html class. Used only in Dartium.
+//wrap_jso(jsObject) => jsObject;
+
+/// Find the underlying JS object for a dart:html Dart object.
+//unwrap_jso(dartClass_instance) => dartClass_instance;