Reapply "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]));
+}