Add Object.hash and Object.hashAll static helper methods.

Fixes #11617.

Bug: http://dartbug.com/11617
Change-Id: Id06fb5b3914bee24713427edbd3b9b7e86f86449
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/73360
Commit-Queue: Lasse R.H. Nielsen <[email protected]>
Reviewed-by: Nate Bosch <[email protected]>
Reviewed-by: Stephen Adams <[email protected]>

Author: lrhn

CHANGELOG.md

@@ -27,6 +27,10 @@
     daylight saving changes that are not precisely one hour.
     (No change on the Web which uses the JavaScript `Date` object.)
 
+*   Adds static methods `hash`, `hashAll` and `hashAllUnordered` to the
+    `Object` class. These can be used to combine the hash codes of
+    multiple objects in a consistent way.
+
 #### `dart:ffi`
 
 *   Adds the `DynamicLibrary.providesSymbol` function to check whether a symbol