dart2dart: Preserve variable names throughout the IR.

tests/language/rewrite_while_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";
 
 
 baz() {}
 
 loop1(x) {
@@ skipping 20 matching lines @@
     if (x < 100) {
         while (n < x) {
             n = n + 1;
             baz();
         }
     }
     baz();
     return n;
 }
 
+loop4(x) {
+    var n = 0;
+    if (x < 100) {
+        while (n < x) {
+            baz();
+            n = n + 1;
+        }
+    }
+    baz();
+    return n;
+}
+
 f1(b) {
   while (b)
     return 1;
 
   return 2;
 }
 
 f2(b) {
   while (b) {
     return 1;
   }
   return 2;
 }
 
 main() {
     Expect.equals(0,  loop1(-10));
     Expect.equals(10, loop1(10));
 
     Expect.equals(0,  loop2(-10));
     Expect.equals(10, loop2(10));
     Expect.equals(0,  loop2(200));
 
     Expect.equals(0,  loop3(-10));
     Expect.equals(10, loop3(10));
     Expect.equals(0,  loop3(200));
 
+    Expect.equals(0,  loop4(-10));
+    Expect.equals(10, loop4(10));
+    Expect.equals(0,  loop4(200));
+
     Expect.equals(1, f1(true));
     Expect.equals(2, f1(false));
     Expect.equals(1, f2(true));
     Expect.equals(2, f2(false));
 }