Fix for the js-interop crash

tests/compiler/dart2js/interop_anonymous_unreachable_test.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.
+
+library tests.dart2js.interop_anonymous_unreachable_test;
+
+import 'dart:async';
+
+import 'package:test/test.dart';
+import 'compiler_helper.dart';
+
+
+main() {
+  test("unreachable code doesn't crash the compiler", () async {
+    // This test is a regression for Issue #24974
+    String generated = await compile("""
+        import 'package:js/js.dart';
+
+        @JS() @anonymous
+        class UniqueLongNameForTesting_A {
+          external factory UniqueLongNameForTesting_A();
+        }
+        main() {}
+    """, returnAll: true);
+
+    // the code should not be included in the output either.
+    expect(generated, isNot(contains("UniqueLongNameForTesting_A")));
+  });
+
+  test('interop types can be tree-shaken', () async {
+    String generated = await compile("""
+        import 'package:js/js.dart';
+
+        // reachable and allocated
+        @JS() @anonymous
+        class UniqueLongNameForTesting_A {
+          external bool get x;
+          external UniqueLongNameForTesting_D get d;
+          external UniqueLongNameForTesting_E get e;
+          external factory UniqueLongNameForTesting_A(
+              {UniqueLongNameForTesting_B arg0});
+        }
+
+        // visible through the parameter above, but not used.
+        @JS() @anonymous
+        class UniqueLongNameForTesting_B {
+          external factory UniqueLongNameForTesting_B();
+        }
+
+        // unreachable
+        @JS() @anonymous
+        class UniqueLongNameForTesting_C {
+          external factory UniqueLongNameForTesting_C();
+        }
+
+        // visible and reached through `d`.
+        @JS() @anonymous
+        class UniqueLongNameForTesting_D {
+          external factory UniqueLongNameForTesting_D();
+        }
+
+        // visible through `e`, but not reached.
+        @JS() @anonymous
+        class UniqueLongNameForTesting_E {
+          external factory UniqueLongNameForTesting_E();
+        }
+
+        main() {
+          print(new UniqueLongNameForTesting_A().x);
+          print(new UniqueLongNameForTesting_A().d);
+        }
+    """, returnAll: true);
+    expect(generated, contains("UniqueLongNameForTesting_A"));
+    expect(generated, contains("UniqueLongNameForTesting_D"));
+    expect(generated, isNot(contains("UniqueLongNameForTesting_B")));
+    expect(generated, isNot(contains("UniqueLongNameForTesting_C")));
+    expect(generated, isNot(contains("UniqueLongNameForTesting_E")));
+  });
+}