It’s been reported elsewhere, and even on the MXMLC Ant task page over at Adobe Labs, but I think it’s worth repeating this major WTF:
The MXMLC Ant task doesn’t fail on a build error.
[mxmlc] Tests.mxml(30): Error: Syntax error: A string literal must be terminated before the line break.
[mxmlc] this isn't even proper code
Why? Why? Why?