Fix unparsing of prefix increments.

tests/compiler/dart2js/unparser_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.
 
 #import('dart:uri');
 #import('parser_helper.dart');
 #import("../../../lib/compiler/compiler.dart");
 #import("../../../lib/compiler/implementation/tree/tree.dart");
 
 testUnparse(String statement) {
@@ skipping 70 matching lines @@
 testIndexedOperatorDecl() {
   testUnparseMember('operator[](int i)=> null;');
 }
 
 testNativeMethods() {
   testUnparseMember('foo()native;');
   testUnparseMember('foo()native "bar";');
   testUnparseMember('foo()native "this.x = 41";');
 }
 
+testPrefixIncrements() {
+  testUnparse('++i;');
+  testUnparse('++a[i];');
+  testUnparse('++a[++b[i]];');
+}
+
 testSimpleFileUnparse() {
   final src = '''
 should_be_dropped() {
 }
 
 should_be_kept() {
 }
 
 main() {
   should_be_kept();
 }
 ''';
   testDart2Dart(src, (String s) {
     Expect.equals('should_be_kept(){}main(){should_be_kept();}', s);
   });
 }
 
 main() {
   testGenericTypes();
   testForLoop();
   testEmptyList();
   testClosure();
   testIndexedOperatorDecl();
   testNativeMethods();
+  testPrefixIncrements();
   testSimpleFileUnparse();
 }