isElementFromByteCode ignores whether Element is from source code

org.checkerframework.javacutil.ElementUtils#isElementFromByteCode(Element) is documented as:

Returns true if the element is declared in ByteCode.

This method appears to be buggy. It returns true if the element can be found in byte code on the classpath.

However, it is possible that an element is was read from source code even though it is also available as byte code from the classpath. For example, a .java file could be supplied on the command line even though the corresponding .class file is also on the classpath. In this case, javac ignores the .class file on the classpath and only uses the source code.

isElementFromByteCode should do the same as javac, and return false if the element has been read from source code.

This issue is affecting Paulo's interprocedural type inference.

Start at org.checkerframework.javacutil.ElementUtils#isElementFromByteCode and inspect how javac distinguishes source-backed elements from classpath bytecode. Reproduce the source-and-classpath case described in the issue, then verify the method returns false for the source element and true for a bytecode element.