Avoid generating duplicate stubs. Fixes issue 3184.

tests/compiler/dart2js/no_duplicate_stub_test.dart

+// Copyright (c) 2012, 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("compiler_helper.dart");
+
+const String TEST = r"""
+class A {
+  foo([a, b]) {}
+}
+
+class B extends A {
+}
+
+main() {
+  var a = [bar, baz];
+  a[0](new A());
+  a[1](new A());
+}
+
+bar(a) {
+  if (a is A) a.foo(a: 42);
+}
+
+baz(a) {
+  if (a is B) a.foo(a: 42);
+}
+""";
+
+main() {
+  String generated = compileAll(TEST);
+  RegExp regexp = new RegExp('foo\\\$1\\\$a: function');
+  Iterator<Match> matches = regexp.allMatches(generated).iterator();
+  checkNumberOfMatches(matches, 1);
+}