Show compiler warnings for dangerous use of this

[dreamofabear]

Property flattening in Closure compiler can cause bugs when using this in non-constructor, non-prototype methods, e.g. this bug due to compiling non-conformant 3P code.

Closure compiler prints warnings for this case that look like this:

"WARNING - dangerous use of this in static method foo.bar"

For some reason, these warnings aren't being displayed during gulp dist -- we should fix that.