Reproduce problem where error disappeared

When the javadoc tool fails with an error that could not be parsed,
the error is silently dropped.

internal/zinc-compile-core/src/test/scala/sbt/internal/inc/javac/CollectingReporter.scala

@@ -0,0 +1,19 @@
+package sbt
+package internal
+package inc
+package javac
+
+import xsbti.Reporter
+import xsbti.Problem
+
+class CollectingReporter extends Reporter {
+  var problems: Array[Problem] = Array[Problem]()
+
+  def reset() = problems = Array[Problem]()
+  def hasErrors() = ???
+  def hasWarnings(): Boolean = ???
+  def printSummary(): Unit = ???
+  def log(problem: xsbti.Problem): Unit = problems :+= problem
+  def comment(pos: xsbti.Position, msg: String): Unit = ???
+
+}

internal/zinc-compile-core/src/test/scala/sbt/internal/inc/javac/JavacProcessLoggerSpec.scala

@@ -0,0 +1,20 @@
+package sbt
+package internal
+package inc
+package javac
+
+import java.io.File
+
+class JavaProcessLoggerSpec extends UnitSpec {
+  "The javac process logger" should "log errors that could not be parsed" in logUnparsableErrors()
+
+  def logUnparsableErrors() = {
+    val reporter = new CollectingReporter()
+    val logger = new JavacLogger(log = null, reporter, cwd = new File("."))
+    logger.err("javadoc: error - invalid flag: -target")
+
+    logger.flush(-1)
+
+    reporter.problems.length should be(1)
+  }
+}