dart2dart: Test cases for better code coverage in dart_tree rewritings.

tests/language/rewrite_assign_test.dart

 // Copyright (c) 2014, 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.
 
 import "package:expect/expect.dart";
 
-// Regression test for Issue 16407.
+bar(x,y) {}
 
-void main() {
-  foo(null, true);
-  foo('x', false);
+foo(b) {
+  var x, y;
+  if (b) {
+    x = 1;
+    y = 2;
+  } else {
+    x = 2;
+    y = 1;
+  }
+  bar(x,y);
+  bar(x,y);
+  return x;
 }
 
-var foo = (x, result) {
-  Expect.equals(result, x is Null, '$x is Null');
-};
+main() {
+    Expect.equals(1, foo(true));
+    Expect.equals(2, foo(false));
+}