dart2dart: Preserve variable names throughout the IR.

tests/language/rewrite_swap.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";
+
+swap1(x,y,b) {
+    if (b) {
+        var t = x;
+        x = y;
+        y = t;
+    }
+    Expect.equals(2, x);
+    Expect.equals(1, y);
+}
+
+swap2(x,y,z,w,b) {
+    if (b) {
+        var t = x;
+        x = y;
+        y = t;
+
+        var q = z;
+        z = w;
+        w = q;
+    }
+    Expect.equals(2, x);
+    Expect.equals(1, y);
+    Expect.equals(4, z);
+    Expect.equals(3, w);
+}
+
+swap3(x,y,z,b) {
+    if (b) {
+        var t = x;
+        x = y;
+        y = z;
+        z = t;
+    }
+    Expect.equals(2, x);
+    Expect.equals(3, y);
+    Expect.equals(1, z);
+}
+
+swap4(x,y,z,b) {
+    if (b) {
+        var t = x;
+        x = y;
+        y = z; // swap cycle involves unused variable 'y'
+        z = t;
+    }
+    Expect.equals(2, x);
+    Expect.equals(1, z);
+}
+
+main() {
+    swap1(1,2,true);
+    swap1(2,1,false);
+
+    swap2(1,2,3,4,true);
+    swap2(2,1,4,3,false);
+
+    swap3(1,2,3,true);
+    swap3(2,3,1,false);
+
+    swap4(1,2,3,true);
+    swap4(2,3,1,false);
+}