Support type promotion in dart2js.

tests/language/type_promotion_logical_and_test.dart

Index: tests/language/type_promotion_logical_and_test.dart
diff --git a/tests/language/type_promotion_logical_and_test.dart b/tests/language/type_promotion_logical_and_test.dart
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index 0000000000000000000000000000000000000000..92a4758a02b8c9e9ad27b5ab9500a2ee6ef0d167
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+++ b/tests/language/type_promotion_logical_and_test.dart
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+// 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 locals potentially mutated.
+
+class A {
+  var a = true;
+}
+class B extends A {
+  var b = true;
+}
+class C extends B {
+  var c = true;
+}
+class D extends A {
+  var d = true;
+}
+class E implements C, D {
+  var a = true;
+  var b = true;
+  var c = true;
+  var d = true;
+}
+
+void main() {
+  A a = new E();
+  var b;
+  if (a is D && ((a = new D()) != null)) {
+    b = a.d; /// 01: static type warning
+  }
+  if (a is D && (b = a.d)) {
+    b = a.d; /// 02: static type warning
+    a = null;
+  }
+  if (f(a = null) && a is D) {
+    b = a.d;
+  }
+}
+
+bool f(x) => true;