code: function mergeDomains( domainAId, domainBId ){
  
          var domainA = domains[ domainAId ];
          var domainB = domains[ domainBId ];
  
          if( domainA.identifier == domainB.identifier ){
              // 更新 domainB 的domainId
  
              domainB.domainId = domainA.domainId;
  
              domainB.points.forEach(function( point ){
                  point.domainId = domainA.domainId;
                  pointsHash[ point.x + '_' + point.y ].domainId = domainA.domainId;
              });
  
              domainA.points = domainA.points.concat( domainB.points );
  
              // 删除domainB
              delete domains[ domainBId ];
          }
      }
docs: 将 domainB 合并到 domainA
  @param domainAId
  @param domainBId