Reported by Google:
Removing a source file in the main sources breaks incremental compilation of the test sources.

Reproduction Steps

You just need a project with
```
build.gradle:
apply plugin: 'java'
configure([compileJava, compileTestJava])

```

and two valid java files in src/main/java and one in src/test/java.
compile, and then remove one of the files from src/main/java

`gradlew build` output:
```
: Incremental java compilation is an incubating feature.
:compileJava
None of the classes needs to be compiled! Analysis took 0.006 secs.
:processResources UP-TO-DATE
:classes
:compileTestJava FAILED

FAILURE: Build failed with an exception.

• What went wrong:
  Execution failed for task ':compileTestJava'.
  > Unable to read class file: '/usr/local/google/home/cmw/sample/build/classes/main/com/example/Foo.class'

• Try:
  Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

BUILD FAILED

Total time: 0.851 secs
```