Switch ddc to use @JS instead of @JSName

test/codegen/expect/js/js.js

+dart_library.library('js/js', null, /* Imports */[
+  "dart/_runtime",
+  'dart/js',
+  'dart/core'
+], /* Lazy imports */[
+], function(exports, dart, js, core) {
+  'use strict';
+  let dartx = dart.dartx;
+  dart.export(exports, js, ['allowInterop', 'allowInteropCaptureThis'], []);
+  class JS extends core.Object {
+    JS(name) {
+      if (name === void 0)
+        name = null;
+      this.name = name;
+    }
+  }
+  dart.setSignature(JS, {
+    constructors: () => ({JS: [JS, [], [core.String]]})
+  });
+  class _Anonymous extends core.Object {
+    _Anonymous() {
+    }
+  }
+  dart.setSignature(_Anonymous, {
+    constructors: () => ({_Anonymous: [_Anonymous, []]})
+  });
+  let anonymous = dart.const(new _Anonymous());
+  // Exports:
+  exports.JS = JS;
+  exports.anonymous = anonymous;
+});