Drop trailing null arguments in generated JS.

tests/compiler/dart2js/drop_trailing_null_arguments_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.
+// Test that parameters keep their names in the output.
+
+import 'package:async_helper/async_helper.dart';
+import 'package:expect/expect.dart';
+import 'compiler_helper.dart';
+
+main() {
+  // If a function call has trailing 'null' arguments, we can safely skip
+  // them as JavaScript will fill missing arguments with 'undefined'.
+  asyncTest(() => compile(r"""
+      class It<A, B> {
+        It([a, b, c]) : super;
+      }
+
+      foo(a,b,c) => a;
+      test() {
+        foo(1, null, null);
+        foo(2, 3, null);
+        new It();
+        new It(null, 1);
+      }
+      """,
+      entry: 'test', minify: false).then((String generated) {
+    Expect.isTrue(generated.contains(r"foo(1)"));
+    Expect.isTrue(generated.contains(r"foo(2, 3)"));
+    Expect.isTrue(generated.contains(r"It$()"));
+    Expect.isTrue(generated.contains(r"It$(null, 1)"));
+  }));
+}