Revert "Refactor type propagation."

tests/language/src/ListDoubleIndexInLoop2Test.dart

-// 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.
-// Dart test program for testing arrays.
-
-bar() => true;
-
-tata() => 1.5;
-
-// The type propagation in Dart2Js wrongly took the intersection of all incoming
-// types in a loop-phi. In this case the back-edge brought type 'number' which,
-// combined with 'integer' (i = 0) was narrowed to 'integer'. As a result no
-// check was inserted for the list access.
-foo(a) {
-  var i;
-  if (bar()) {
-    // t's desired type is conflicting. Once it is used as array receiver. And
-    // once as integer. The backward propagation thus can't decide.
-    // The forward declaration, however, will assign type num.
-    var t = 0 + tata();
-    i = t;
-    if (!bar()) t[0];
-  } else {
-    i = 0;
-  }
-  // The phi, combining the two 'i's must reach the conclusion that i is of
-  // type num and therefore needs a check before accessing the array.
-  a[i];
-}
-
-main() {
-  Expect.throws(() => foo([1, 2]));
-}