Dave Jones' uCurrent Gold - trace to shunt resistors has no soldermask
I wanted to answer my own question in case someone is interested. This bottleneck was a fusible track. The EEVBlog forum had a discussion on this. 

The idea of the trace is that it would fuse and protect some of the components on the PCB in case of a very large current input. The temperature rise of the fuse seems to be dictate by the trace width, material used for plating and the substrate surrounding the same. However, this is not a replacement for a standalone fuse. 

Additional reading on fusible tracks: 
http://www.cetti.ro/cadence/articles/New_Methods_of_Testing_PCB_Traces_Capacity_and_Fusing.pdf