Added full js & js_util packages

packages/js_util/src/js_util_base.dart

-@JS()
-library js_util.base;
-
-import 'package:js/js.dart';
-import 'package:quiver_iterables/iterables.dart';
-
-// js_util library uses the technique described in
-// https://github.com/dart-lang/sdk/issues/25053
-// because dart2js compiler does not support [] operator.
-
-@JS()
-@anonymous
-class PropertyDescription {
-  external factory PropertyDescription(
-      {bool configurable, bool enumerable, value, bool writable});
-}
-
-/// A wrapper for https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty
-@JS('Object.defineProperty')
-external void defineProperty(o, String prop, PropertyDescription description);
-
-/// Returns `o[prop]`.
-@JS('JsUtil.getValue')
-external getValue(o, String prop);
-
-/// Creates a new JavaScript object.
-@JS('JsUtil.newObject')
-external newObject();
-
-/// Performs `o[key] = value`.
-void setValue(o, String key, value) {
-  defineProperty(o, key, new PropertyDescription(value: value));
-}
-
-/// Converts a Dart object to a JavaScript object.
-///
-/// For example, you can convert a Dart [Map] to a JavaScript object.
-///
-///     final jsObj = toJS({
-///       'people': [
-///         {'firstName': 'Kwang Yul', 'lastName': 'Seo'},
-///         {'firstName': 'DoHyung', 'lastName': 'Kim'},
-///         {'firstName': 'Kyusun', 'lastName': 'Kim'}
-///       ]
-///     });
-///
-toJS(o) {
-  if (o is Map) {
-    final newObj = newObject();
-    for (final keyValuePair in zip([o.keys, o.values])) {
-      setValue(newObj, keyValuePair[0], toJS(keyValuePair[1]));
-    }
-    return newObj;
-  } else if (o is List) {
-    return o.map((e) => toJS(e)).toList();
-  } else {
-    return o;
-  }
-}