Some people might find this useful.
When your compressing your files into app-all.js, sencha build just invokes YUI Compressor. The issue is that the sencha build invocation hides the output from YUI Compressor, so if something goes wrong you're screwed.
If this happens to you, you need to snag the yuicompressor.jar yourself, and try to build it
java -jar ycompressor.jar all-classes.js -o app-all.js
When you run this command, ycompressor will tell you what's wrong.
The output will look like the following, and then you can go see what's wrong with your all-classes.js
PM\YCompressor\ycompressor.jar all-classes.js -o app-all.js
[ERROR] 71165:34:missing name after . operator
[ERROR] 71166:26yntax error
[ERROR] 71167:28yntax error
[ERROR] 71168:28yntax error
[ERROR] 71169:26yntax error
[ERROR] 71172:22:invalid property id
[ERROR] 71172:47yntax error
[ERROR] 71175:22:missing ; before statement
[ERROR] 71176:23yntax error
[ERROR] 71177:25yntax error