We knew there was a compiler bug in Flash Builder 4 in previous article Flash Builder 4 Compiler Bug ( FB 4.0 build 272416 ) And Solutions and listed all compiler options of mxmlc, we have known how to avoid that issue, but it isn’t the end, oppositely only a begin.

Because Ticore continue his digging and found the farther Flex SDK 4 Omit Trace Bug(Chinese Traditional), if

-compiler.debug=false -compiler.omit-trace-statements=true

be set in the compiler argument, it would cause the flex SDK compiler setting in disorder—all “trace()” would be skipped, and couldn’t back to debug mode, no matter modify the compiler setting or not.

Or if you only set:

-compiler.debug=false

the same issue would be caused because

-compiler.omit-trace-statements=true

was set on flex-config.xml as a default.

Ticore also provided two solutions:

add -compiler.omit-trace-statements=false into compiler arguments.
edit flex-config.xml, change omit-trace-statements to false

Although Flex 4 SDK and Flash Builder 4 was released after more than six months in BETA, but some details like above not be paid adequate attention.