Added js and js_util to observatory deps

packages/js/src/varargs.dart

+// 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.
+
+/// Declarations for variable arguments support
+/// (rest params and spread operator).
+///
+/// These are currently *not* supported by dart2js or Dartium.
+library js.varargs;
+
+class _Rest {
+  const _Rest();
+}
+
+/// Annotation to tag ES6 rest parameters (https://goo.gl/r0bJ1K).
+///
+/// This is *not* supported by dart2js or Dartium (yet).
+///
+/// This is meant to be used by the Dart Dev Compiler
+/// when compiling helper functions of its runtime to ES6.
+///
+/// The following function:
+///
+///     foo(a, b, @rest others) { ... }
+///
+/// Will be compiled to ES6 code like the following:
+///
+///     function foo(a, b, ...others) { ... }
+///
+/// Which is roughly equivalent to the following ES5 code:
+///
+///     function foo(a, b/*, ...others*/) {
+///       var others = [].splice.call(arguments, 2);
+///       ...
+///     }
+///
+const _Rest rest = const _Rest();
+
+/// Intrinsic function that maps to the ES6 spread operator
+/// (https://goo.gl/NedHKr).
+///
+/// This is *not* supported by dart2js or Dartium (yet),
+/// and *cannot* be called at runtime.
+///
+/// This is meant to be used by the Dart Dev Compiler when
+/// compiling its runtime to ES6.
+///
+/// The following expression:
+///
+///     foo(a, b, spread(others))
+///
+/// Will be compiled to ES6 code like the following:
+///
+///     foo(a, b, ...others)
+///
+/// Which is roughly equivalent to the following ES5 code:
+///
+///     foo.apply(null, [a, b].concat(others))
+///
+dynamic spread(args) {
+  throw new StateError(
+      'The spread function cannot be called, '
+      'it should be compiled away.');
+}