Support percent/permille formats with positive and negative variations

pkg/intl/test/number_format_test.dart

Index: pkg/intl/test/number_format_test.dart
diff --git a/pkg/intl/test/number_format_test.dart b/pkg/intl/test/number_format_test.dart
index 83d53d89b9522c4c749add1df1cbce74c530c587..1ff898b2c1a01ac0c1037bd5ea41d026773ece6c 100644
--- a/pkg/intl/test/number_format_test.dart
+++ b/pkg/intl/test/number_format_test.dart
@@ -187,6 +187,12 @@ main() {
     expect(readBack, amount);
   });
 
+  test("Delta percent format", () {
+    var f = new NumberFormat("+#,##0%;-#,##0%");
+    expect(f.format(-0.07), "-7%");
+    expect(f.format(0.12), "+12%");
+  });
+
   test('Unparseable', () {
     var format = new NumberFormat.currencyPattern();
     expect(() => format.parse("abcdefg"), throwsFormatException);