Fix various warnings from the analyzer.

sdk/lib/core/date_time.dart

 // Copyright (c) 2011, 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.
 
 part of dart.core;
 
 /**
  * Deprecated class. Please use [DateTime] instead.
  */
 abstract class Date implements Comparable {
@@ skipping 81 matching lines @@
   bool get isUtc;
 
   String toString();
 
   DateTime add(Duration duration);
   DateTime subtract(Duration duration);
   Duration difference(DateTime other);
 }
 
 /**
- * Date is the public interface to a point in time.
+ * A DateTime object represents a point in time.
  *
  * It can represent time values that are at a distance of at most
  * 8,640,000,000,000,000ms (100,000,000 days) from epoch (1970-01-01 UTC). In
  * other words: [:millisecondsSinceEpoch.abs() <= 8640000000000000:].
  *
  * Also see [Stopwatch] for means to measure time-spans.
  */
 class DateTime implements Date {
   // Weekday constants that are returned by [weekday] method:
   static const int MON = 1;
@@ skipping 354 matching lines @@
    * Returns the millisecond into the second [0...999].
    */
   external int get millisecond;
 
   /**
    * Returns the week day [MON..SUN]. In accordance with ISO 8601
    * a week starts with Monday which has the value 1.
    */
   external int get weekday;
 }