Remove js.Sequence and use binary comma operator instead.

tests/compiler/dart2js/js_parser_statements_test.dart

 // Copyright (c) 2014, 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.
 
 import 'package:expect/expect.dart';
 import 'mock_compiler.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/js/js.dart' as jsAst;
 import '../../../sdk/lib/_internal/compiler/implementation/js/js.dart' show js;
 
 
@@ skipping 124 matching lines @@
 
   testStatement('#.prototype.# = function(#) { return #.# };',
       ['className', 'getterName', ['r', 'y'], 'r', 'fieldName'],
       'className.prototype.getterName = function(r, y) {\n'
       '  return r.fieldName;\n'
       '};');
 
   testStatement('function foo(r, #) { return #[r](#) }',
       [['a', 'b'], 'g', ['b', 'a']],
       'function foo(r, a, b) {\n  return g[r](b, a);\n}');
+
+  // Sequence is printed flattened
+  var seq1 = js('1, 2, 3');
+  testStatement('x = #', [seq1], 'x = (1, 2, 3);');
+  testStatement('x = (#, #)', [seq1, seq1], 'x = (1, 2, 3, 1, 2, 3);');
+  testStatement('x = #, #', [seq1, seq1], 'x = (1, 2, 3), 1, 2, 3;');
+  testStatement(
+      'for (i = 0, j = #, k = 0; ; ++i, ++j, ++k){}', [seq1],
+      'for (i = 0, j = (1, 2, 3), k = 0;; ++i, ++j, ++k) {\n}');
 }