This repository contains the source code for the Dragon 64 versions of the Microsoft 16K BASIC Interpreter for the Motorola 6809 (aka BASIC-69 and Extended Color BASIC).
Small correction (I think), you conflate Extended Color Basic with BASIC-09. The latter was not a Microsoft product, but was a structured basic made by Microware, the creator of OS-9: https://en.wikipedia.org/wiki/BASIC09
ECBCOM.SA seems like it should be a Microsoft file, but we don't have any direct source for it. The listing starts each source file with the header showing ECBCOM.SA on drive 0 before moving to the real name (like ECBRAM.SA) on drive 1. Best guess is that ECBCOM defines common macros like SKIP2 and then includes each file (or each file includes ECBCOM.SA as the first thing it does).
BTW the listing glosses over what is being output for the likes of SKIP2! There are similar if not identical macros in the DragonDOS 2.0c source, so those could be substituted - for creating source that actually assembles, not for publishing as Microsoft source - if they look like they fit the address spans and CC bit expectation.