Add DateTime.replace.

sdk/lib/core/date_time.dart

Index: sdk/lib/core/date_time.dart
diff --git a/sdk/lib/core/date_time.dart b/sdk/lib/core/date_time.dart
index 28ece3fc8289e80cf2a9bd00f23375ff3ba7d5fb..e44e419ec522e2e394059f197928c1ab3f9c65a3 100644
--- a/sdk/lib/core/date_time.dart
+++ b/sdk/lib/core/date_time.dart
@@ -371,6 +371,38 @@ class DateTime implements Comparable {
   }
 
   /**
+   * Replaces specific parts of a date or time while keeping the rest.
+   *
+   * Equivalent to createing a new `DateTime` using `new DateTime`
+   * with the values specified as arguments to this method, and taking
+   * the values of the this `DateTime` object for arguments that were omitted.
+   *
+   * # Example
+   *
+   *     var dt = new DateTime(1969, 7, 20, 20, 18);
+   *     var ut = dt.replace(day: 21, hour: 17, minute: 54);
+   *     Expect.equals("1969-07-21T17:54:00.000", "$ut");
+   *
+   */
+  DateTime replace({int year,
+                    int month,
+                    int day,
+                    int hour,
+                    int minute,
+                    int second,
+                    int millisecond,
+                    bool isUtc}) {
+    return new DateTime._internal(year        ?? this.year,
+                                  month       ?? this.month,
+                                  day         ?? this.day,
+                                  hour        ?? this.hour,
+                                  minute      ?? this.minute,
+                                  second      ?? this.second,
+                                  millisecond ?? this.millisecond,
+                                  isUtc       ?? this.isUtc);
+  }
+
+  /**
    * Returns true if [other] is a [DateTime] at the same moment and in the
    * same time zone (UTC or local).
    *