Remove deprecated string features.

samples/swarm/swarm_ui_lib/layout/GridTracks.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 layout;
 
 // This file has classes representing the grid tracks and grid template
 
 /**
  * The data structure representing the grid-rows or grid-columns
@@ skipping 100 matching lines @@
     : _rects = new Map<int, _GridTemplateRect>(),
       _numRows = rows.length {
     _buildRects(rows);
   }
 
   /** Scans the template strings and computes bounds for each one. */
   void _buildRects(List<String> templateRows) {
     for (int r = 0; r < templateRows.length; r++) {
       String row = templateRows[r];
       for (int c = 0; c < row.length; c++) {
-        int cell = row.charCodeAt(c);
+        int cell = row.codeUnitAt(c);
         final rect = _rects[cell];
         if (rect != null) {
           rect.add(r + 1, c + 1);
         } else {
           _rects[cell] = new _GridTemplateRect(cell, r + 1, c + 1);
         }
       }
     }
 
     // Finally, check that each rectangle is valid (i.e. all spaces filled)
     for (final rect in _rects.values) {
       rect.checkValid();
     }
   }
 
   /**
    * Looks up the given cell in the template, and returns the rect.
    */
   _GridTemplateRect lookupCell(String cell) {
     if (cell.length != 1) {
       throw new UnsupportedError(
           'grid-cell "$cell" must be a one character string');
     }
-    final rect = _rects[cell.charCodeAt(0)];
+    final rect = _rects[cell.codeUnitAt(0)];
     if (rect == null) {
       throw new UnsupportedError(
         'grid-cell "$cell" not found in parent\'s grid-template');
     }
     return rect;
   }
 }
 
 /** Used by GridTemplate to track a single cell's bounds. */
 class _GridTemplateRect {
@@ skipping 24 matching lines @@
 /**
  * Used to return a row/column and span during parsing of grid-row and
  * grid-column during parsing.
  */
 class _GridLocation {
   final int start, length;
   _GridLocation(this.start, this.length) {}
 
   int get end => start + length;
 }