The throttle control or carburetor linkage isn't stuck, is it? For a small two stroke engine to rev up high like that it would also need a lot of air to go along with all that fuel. If the engine was enriched at idle with enough fuel to run it at full speed without the excess air, it would most likely die due to the air/fuel ratio being so far to the rich side (not enough oxygen for the amount of fuel). If the engine had a large air leak such as a bad carburetor to engine mount gasket, it would be hard to start. The typical Stihl chainsaw has a four position master control switch and you have to depress the top throttle lock on the handle and pull the throttle to wide open the close the choke plate by moving that master control switch to the most downward position. This also sets the carburetor to a fast idle position. The starting procedure is as follows: set choke, and pull starter rope several times or until the engine "burps" (attempts to start) then move master control switch up one position to the fast idle position and continue pulling the starter rope until the engine starts. After the engine starts you are to "blip" the throttle which automatically moves the carburetor linkage back to a normal idle. If for some reason your throttle trigger or carburetor linkage is not working properly it could cause the condition you are describing. I'm sure you know how to start your chainsaw but I wrote out the instructions just in case I missed something.
You can always take your saw to the Stihl dealer and they will most likely take a look for free to see if anything is not functioning properly. Most dealers are willing to at least take a look and try to get you back in service asap so it's worth a try.