Fix a bug in type assertions involving String supertypes.

dart/tests/language/string_supertype_checked_test.dart

Index: dart/tests/language/string_supertype_checked_test.dart
diff --git a/dart/tests/language/string_supertype_checked_test.dart b/dart/tests/language/string_supertype_checked_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..171e31a539663718b424f45c0a367266d24bd934
--- /dev/null
+++ b/dart/tests/language/string_supertype_checked_test.dart
@@ -0,0 +1,27 @@
+// 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.
+
+// This tests a bug in dart2js which caused the compiler to emit bad
+// type assertions for supertypes of String.
+
+class A implements Comparable {
+  int value;
+
+  A(this.value);
+
+  int compareTo(Comparable other) {
+    A o = promote(other);
+    return value.compareTo(o.value);
+  }
+
+  A promote(var other) {
+    return other;
+  }
+}
+
+main() {
+  var a = new A(1);
+  var b = new A(2);
+  Expect.equals(-1, a.compareTo(b));
+}