dart2dart: Preserve variable names throughout the IR.

tests/language/rewrite_while_test.dart

Index: tests/language/rewrite_while_test.dart
diff --git a/tests/language/rewrite_while_test.dart b/tests/language/rewrite_while_test.dart
index 18c6539318561e2600d5def9fcbef69d3f03802d..e7375ef927a165711fadff26574d24c72df1156c 100644
--- a/tests/language/rewrite_while_test.dart
+++ b/tests/language/rewrite_while_test.dart
@@ -38,6 +38,18 @@ loop3(x) {
     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;
@@ -64,6 +76,10 @@ main() {
     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));