Add support for DateTimeFormat.formatToParts

test/intl/date-format/date-format-to-parts.js

+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --datetime-format-to-parts
+
+var d = new Date(2016, 11, 15, 14, 10, 34);
+var df = Intl.DateTimeFormat("ja",
+    {hour: 'numeric', minute: 'numeric', second: 'numeric', year: 'numeric',
+     month: 'numeric', day: 'numeric', timeZoneName: 'short', era: 'short'});
+
+var formattedParts = df.formatToParts(d);
+
+var formattedReconstructedFromParts = formattedParts.map((part) => part.value)
+    .reduce((accumulated, part) => accumulated + part);
+assertEquals(df.format(d), formattedReconstructedFromParts);
+// 西暦2016年11月15日 14:10:34 GMT-7
+assertEquals(["era", "year", "literal", "month", "literal", "day", "literal",
+              "hour", "literal", "minute", "literal", "second", "literal",
+              "timeZoneName"], formattedParts.map((part) => part.type));