Dot convention in Magnetically coupled circuits
The dot only determines the polarity of the mutual voltage, it won't affect other directions in the calculation. In figure (b), it shows the equivalent circuit take into the mutual voltage. Because the direction of \$I_{1}\$ in loop 1 is flow into coil 1, and the direction of \$I_2\$ is defined to flow out from coil 2. So the mutual voltage's polarity in loop 2 is positive at the dot side.