Move make_dart_rectangle to html_common

sdk/lib/html/html_common/conversions_dartium.dart

 part of html_common;
 
 convertDartToNative_PrepareForStructuredClone(value) =>
     new _StructuredCloneDartium().convertDartToNative_PrepareForStructuredClone(value);
 
 convertNativeToDart_AcceptStructuredClone(object, {mustCopy: false}) =>
     new _AcceptStructuredCloneDartium().convertNativeToDart_AcceptStructuredClone(object, mustCopy: mustCopy);
 
 class _StructuredCloneDartium extends _StructuredClone {
   newJsMap() => new js.JsObject(js.context["Object"]);
@@ skipping 48 matching lines @@
   var completer = new Completer();
   var newPromise = promise
     .callMethod("then", [(result) => completer.complete(result)])
     .callMethod("catch", [(result) => completer.completeError(result)]);
   return completer.future;
 }
 
 convertDartToNative_DateTime(DateTime date) {
   return new js.JsObject(js.context["Date"], [date.millisecondsSinceEpoch]);
 }
+
+/// Creates a Dart Rectangle from a Javascript object with properties
+/// left, top, width and height. Used internally in Dartium.
+Rectangle make_dart_rectangle(r) =>
+    r == null ? null : new Rectangle(
+    js.JsNative.getProperty(r, 'left'),
+    js.JsNative.getProperty(r, 'top'),
+    js.JsNative.getProperty(r, 'width'),
+    js.JsNative.getProperty(r, 'height'));