Parse javac non-compile-errors in JavaErrorParser

Upon startup, javac may report errors because (for instance) because it
received incorrect flags. These errors were not correctly parsed by the
JavaErrorParser and were never reported to the user.

This commit fixes this problem by adding a new parsing rule in
JavaErrorParser: errors that start with the prefix "javac:" are now
correctly parsed and reported to the user.

Fixes sbt/sbt#2256

internal/incrementalcompiler-compile-core/src/main/scala/sbt/internal/inc/javac/JavaErrorParser.scala

@@ -48,6 +48,7 @@ class JavaErrorParser(relativeDir: File = new File(new File(".").getAbsolutePath
 
   override val skipWhitespace = false
 
+  val JAVAC: Parser[String] = literal("javac")
   val CHARAT: Parser[String] = literal("^")
   val SEMICOLON: Parser[String] = literal(":") | literal("\uff1a")
   val SYMBOL: Parser[String] = allUntilChar(':') // We ignore whether it actually says "symbol" for i18n
@@ -165,8 +166,17 @@ class JavaErrorParser(relativeDir: File = new File(new File(".").getAbsolutePath
         msg
       )
     }
+  val javacError: Parser[Problem] =
+    JAVAC ~ SEMICOLON ~ restOfLine ^^ {
+      case _ ~ _ ~ error =>
+        new JavaProblem(
+          JavaNoPosition,
+          Severity.Error,
+          s"javac:$error"
+        )
+    }
 
-  val potentialProblem: Parser[Problem] = warningMessage | errorMessage | noteMessage
+  val potentialProblem: Parser[Problem] = warningMessage | errorMessage | noteMessage | javacError
 
   val javacOutput: Parser[Seq[Problem]] = rep(potentialProblem)
   /**

internal/incrementalcompiler-compile-core/src/test/scala/sbt/internal/inc/javac/javaErrorParserSpec.scala

@@ -13,6 +13,7 @@ class JavaErrorParserSpec extends UnitSpec {
   "The JavaErrorParser" should "be able to parse linux errors" in parseSampleLinux()
   it should "be able to parse windows file names" in parseWindowsFile()
   it should "be able to parse windows errors" in parseSampleWindows()
+  it should "be able to parse javac errors" in parseSampleJavac()
 
   def parseSampleLinux() = {
     val parser = new JavaErrorParser()
@@ -43,6 +44,14 @@ class JavaErrorParserSpec extends UnitSpec {
     }
   }
 
+  def parseSampleJavac() = {
+    val parser = new JavaErrorParser()
+    val logger = Logger.Null
+    val problems = parser.parseProblems(sampleJavacMessage, logger)
+    problems should have size (1)
+    problems(0).message shouldBe (sampleJavacMessage)
+  }
+
   def sampleLinuxMessage =
     """
       |/home/me/projects/sample/src/main/Test.java:4: cannot find symbol
@@ -61,4 +70,6 @@ class JavaErrorParserSpec extends UnitSpec {
 
   def windowsFile = """C:\Projects\sample\src\main\java\Test.java"""
   def windowsFileAndLine = s"""$windowsFile:4"""
+
+  def sampleJavacMessage = "javac: invalid flag: -foobar"
 }