fix(change-detection): remove memory leak

When the change detection finds change it creates a
linked list of records which have changed. The linked
list is not bound to groups but rather crosses groups, 
this implies that if we remove a group it is possible that
a record in a live group retains a pointer to a 
record in the now deleted record. While technically
not a leak, the memory will not be realized until that
record detects another change and gets re-added to the
dirty list. If the record never fires than the
memory is retained forever.

Author: mhevery

lib/change_detection/watch_group.dart

@@ -415,7 +415,9 @@ class RootWatchGroup extends WatchGroup {
       } catch (e, s) {
         if (exceptionHandler == null) rethrow; else exceptionHandler(e, s);
       }
-      dirtyWatch = dirtyWatch._nextDirtyWatch;
+      var nextDirtyWatch = dirtyWatch._nextDirtyWatch;
+      dirtyWatch._nextDirtyWatch = null;
+      dirtyWatch = nextDirtyWatch;
     }
     _dirtyWatchHead = _dirtyWatchTail = null;
     if (processStopwatch != null) processStopwatch..stop()..increment(count);

lib/core/scope.dart

@@ -453,6 +453,7 @@ class RootScope extends Scope {
           }
           _runAsyncHead = _runAsyncHead._next;
         }
+        _runAsyncTail = null;
 
         digestTTL--;
         count = rootWatchGroup.detectChanges(
@@ -498,6 +499,7 @@ class RootScope extends Scope {
           }
           _domWriteHead = _domWriteHead._next;
         }
+        _domWriteTail = null;
         if (runObservers) {
           runObservers = false;
           observeGroup.detectChanges(exceptionHandler:_exceptionHandler);
@@ -510,6 +512,7 @@ class RootScope extends Scope {
           }
           _domReadHead = _domReadHead._next;
         }
+        _domReadTail = null;
       } while (_domWriteHead != null || _domReadHead != null);
       assert((() {
         var watchLog = [];