Revert "Change some platform libraries to use `part of URI`."

sdk/lib/html/html_common/conversions_dartium.dart

-part of "dart:html_common";
+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 {
@@ skipping 273 matching lines @@
     if (js.JsNative.hasProperty(e, "postMessage")) {
       // assume this is a Window. To match Dart2JS, separate conversion code
       // in dart:html will switch the wrapper to a cross frame window as
       // required.
       // TODO(jacobr): we could consider removing this code completely.
       return Window.instanceRuntimeType;
     }
   }
   return js.JSObject.instanceRuntimeType;
 }