When running mvn clean install, hpi:apt-compile fails on an obviously correct Java file:

[INFO] [hpi:apt-compile

]
[INFO] Compiling 3 source files to C:\work\hudson\hudson\plugins\downstream-ext\
target\classes
Note: Generating hudson\plugins\downstream_ext\DownstreamTrigger.stapler
C:\work\hudson\hudson\plugins\downstream-ext\src\main\java\hudson\plugins\downst
ream_ext\DownstreamTrigger.java:221: package BuildTrigger does not exist
public static class DescriptorImpl extends BuildTrigger.DescriptorImpl {
^
C:\work\hudson\hudson\plugins\downstream-ext\src\main\java\hudson\plugins\downst
ream_ext\DownstreamTrigger.java:241: cannot find symbol
symbol : class FormException
location: class hudson.plugins.downstream_ext.DownstreamTrigger.DescriptorImpl
public Publisher newInstance(StaplerRequest req, JSONObject formData) th
rows FormException {

^
C:\work\hudson\hudson\plugins\downstream-ext\src\main\java\hudson\plugins\downst
ream_ext\DownstreamTrigger.java:218: not assignable to java.lang.Object
@Extension
^
Note: hudson.plugins.downstream_ext.DownstreamTrigger.DescriptorImpl.ItemListene
rImpl indexed under hudson.Extension
1 error
[INFO] ------------------------------------------------------------------------
[ERROR] BUILD ERROR

The offending line seem to be this one:
@Extension
public static class DescriptorImpl extends BuildTrigger.DescriptorImpl {

When I fully qualify BuildTrigger, the build succeeds. I.e.
@Extension
public static class DescriptorImpl extends hudson.tasks.BuildTrigger.DescriptorImpl {

You can try out yourself with revision 27094 of the downstream-ext plugin