Fix type propagation bug by always narrowing types.

tests/language/type_propagation_test.dart

+// 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.
+
+// dart2js used to have an infinite loop in its type propagation
+// algorithm due to types becoming broader instead of narrower.
+
+import "package:expect/expect.dart";
+
+class A {
+  resolveSend(node) {
+    if (node == null) {
+      return [new B()][0];
+    } else {
+      return [new B(), new A()][1];
+    }
+  }
+
+  visitSend(node) {
+    var target = resolveSend(node);
+
+    if (false) {
+      if (false) {
+        target = target.getter;
+        if (false) {
+          target = new Object();
+        }
+      }
+    }
+    return true ? target : null;
+  }
+}
+
+var a = 43;
+
+class B {
+  var getter = a == 42 ? new A() : null;
+}
+
+main() {
+  Expect.isTrue(new A().visitSend(new A()) is A);
+  Expect.isTrue(new A().visitSend(null) is B);
+}