Support type promotion in dart2js.

tests/language/type_promotion_assign_test.dart

Index: tests/language/type_promotion_assign_test.dart
diff --git a/tests/language/type_promotion_assign_test.dart b/tests/language/type_promotion_assign_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..51f0ee5d4b7dc8141fabeec913d56a1da7fbf2bf
--- /dev/null
+++ b/tests/language/type_promotion_assign_test.dart
@@ -0,0 +1,47 @@
+// Copyright (c) 2013, 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 type promotion of assigned locals.
+
+class A {
+  var a = "a";
+}
+class B extends A {
+  var b = "b";
+}
+class C extends B {
+  var c = "c";
+}
+class D extends A {
+  var d = "d";
+}
+class E implements C, D {
+  var a = "";
+  var b = "";
+  var c = "";
+  var d = "";
+}
+
+void main() {
+  A a = new E();
+  if (a is B) {
+    print(a.a);
+    print(a.b); /// 01: static type warning
+    a = null;
+  }
+  if (a is B) {
+    a = null;
+    print(a.a);
+    print(a.b); /// 02: static type warning
+  }
+  if (a is B) {
+    print(a.a);
+    print(a.b); /// 03: static type warning
+    {
+      a = null;
+    }
+    print(a.a);
+    print(a.b); /// 04: static type warning
+  }
+}