dart2dart: Preserve variable names throughout the IR.

tests/language/rewrite_variable_initializer_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";
+
+class Foo {
+    var field = 0;
+}
+
+bar(x,y) {
+    return x * 100 + y;
+}
+
+foo(z) {
+    var x = 0, y = x;
+    if (z > 0) {
+        x = 10;
+    }
+    if (z > 10) {
+        y = 20;
+    }
+    return bar(x,y);
+}
+
+baz(z) {
+    var f = new Foo()
+            ..field = 10
+            ..field = z;
+    return f;
+}
+
+main() {
+    Expect.equals(0, foo(0));
+    Expect.equals(1000, foo(5));
+    Expect.equals(1020, foo(15));
+
+    Expect.equals(20, baz(20).field);
+    Expect.equals(30, baz(30).field);
+}