Fix unparsing of prefix increments.

tests/compiler/dart2js/unparser_test.dart

Index: tests/compiler/dart2js/unparser_test.dart
diff --git a/tests/compiler/dart2js/unparser_test.dart b/tests/compiler/dart2js/unparser_test.dart
index a16d892b3ff73b36a17026e9d167083757a51d04..a810b34caa79d267c299f7eb65ded1f4d077f234 100644
--- a/tests/compiler/dart2js/unparser_test.dart
+++ b/tests/compiler/dart2js/unparser_test.dart
@@ -88,6 +88,12 @@ testNativeMethods() {
   testUnparseMember('foo()native "this.x = 41";');
 }
 
+testPrefixIncrements() {
+  testUnparse('++i;');
+  testUnparse('++a[i];');
+  testUnparse('++a[++b[i]];');
+}
+
 testSimpleFileUnparse() {
   final src = '''
 should_be_dropped() {
@@ -112,5 +118,6 @@ main() {
   testClosure();
   testIndexedOperatorDecl();
   testNativeMethods();
+  testPrefixIncrements();
   testSimpleFileUnparse();
 }