Compute the swap temporary to not clash with parameter names.

dart/tests/language/parameter_name_conflict_test.dart

+// Copyright (c) 2012, 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.
+
+foo(t0) {
+  var a = t0, b = baz(), c = bar();
+  if (t0 == 'foo') {
+    // Force a SSA swapping problem where dart2js used to use 't0' as
+    // a temporary variable.
+    var tmp = c;
+    c = b;
+    b = tmp;
+  }
+
+  Expect.equals('foo', a);
+  Expect.equals('foo', t0);
+  Expect.equals('bar', b);
+  Expect.equals('baz', c);
+}
+
+bar() => 'bar';
+baz() => 'baz';
+
+main() {
+  foo('foo');
+}