Fix issue 12336: there was a bug when updating phis at exit of a block with a HTypeKnown instructio…

tests/language/issue12336_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 generate wrong code for
+// [foo].
+
+import "package:expect/expect.dart";
+import "compiler_annotations.dart";
+
+main() {
+  var result = foo(1, 2);
+  Expect.equals(1, result[0]);
+  Expect.equals(2, result[1]);
+
+  result = foo([], 2);
+  Expect.equals(0, result[0]);
+  Expect.listEquals([], result[1]);
+}
+
+@DontInline()
+foo(a, b) {
+  () => 42;
+  if (a is List) {
+    var saved = a as List;
+    // By having two HTypeKnown SSA instructions for [a], dart2js was
+    // confused when updating the phis at exit of this block.
+    a = a.length;
+    b = saved;
+  }
+  return [a, b];
+}