Compile package:js.{rest, spread} helpers

lib/src/codegen/js_interop.dart

Index: lib/src/codegen/js_interop.dart
diff --git a/lib/src/codegen/js_interop.dart b/lib/src/codegen/js_interop.dart
new file mode 100644
index 0000000000000000000000000000000000000000..4e9cbad9d367a8cd1b19acfa20549a45f25c12c1
--- /dev/null
+++ b/lib/src/codegen/js_interop.dart
@@ -0,0 +1,31 @@
+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library dev_compiler.src.js_interop;
+
+import 'package:analyzer/src/generated/ast.dart';
+import 'package:analyzer/src/generated/element.dart';
+import 'package:analyzer/src/generated/constant.dart';
+
+bool _isJsLibType(String expectedName, Element e) =>
+    e?.name == expectedName && _isJsLib(e.library);
+
+bool _isJsLib(LibraryElement e) {
+  var libName = e?.name;
+  return libName == 'js' ||
+      libName == 'js.varargs' ||
+      libName == 'dart._js_helper';
+}
+
+bool isJsRestAnnotation(DartObjectImpl value) =>
+    _isJsLibType('_Rest', value.type.element);
+
+bool isJsSpreadInvocation(MethodInvocation i) =>
+    _isJsLibType('spread', i.methodName?.bestElement);
+
+// TODO(jmesserly): Move JsName, JsPeerInterface to package:js (see issue #135).
+bool isJsNameAnnotation(DartObjectImpl value) => value.type.name == 'JsName';
+
+bool isJsPeerInterface(DartObjectImpl value) =>
+    value.type.name == 'JsPeerInterface';