Break a cycle in the speculative type propagator by checking if a desired type goes to unknown.

tests/language/type_propagation2_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.
+
+// Regression test for dart2js that used to infinite loop on
+// speculatively propagating types.
+
+class Bar {
+  noSuchMethod(e) => null;
+}
+
+main() {
+  var d = new Bar();
+
+  while (false) {
+    // [input] will change from indexable to unknown: the use line 20
+    // changes its decision because [a2] changes its type from unknown to
+    // null.
+    var input = ((x) {})(null);
+    var p2 = input.keys.firstWhere(null);
+    var a2 = input.keys.firstWhere(null);
+    print(input[a2] == p2);
+  }
+}