anchor.diff:          left = marginValue(exprs.expressions[3]);
           break;
         default:
  -        return;
  +        return null;
       }
   
       return new BoxEdge.clockwiseFromTop(top, right, bottom, left);
  
positive.code:       case TokenKind.DIRECTIVE_CONTENT:
          // TODO(terry): TBD
          _warning("@content not implemented.", _makeSpan(start));
          return;
      }
    }
  
    /**
  
negative.code:                   pseudoPage.name == 'first')) {
                _warning("Pseudo page must be left, top or first",
                    pseudoPage.span);
                return null;
              }
            }
          }
  