SI-6502 More robust REPL :require

  - handle missing files gracefully (rather than NPE)
  - read the class name with ASM, rather than with a dummy
    classloader. The dummy classloader is prone to throwing
    `LinkageError`s, as reported in the comments of SI-6502.

Manual test of the original report:

```
% qscala
Welcome to Scala version 2.11.5-20150115-183424-155dbf3fdf (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_25).
Type in expressions to have them evaluated.
Type :help for more information.

scala> :require does/not/exist
Cannot read: does/not/exist

scala> classOf[org.junit.Test]
<console>:8: error: object junit is not a member of package org
              classOf[org.junit.Test]
                          ^

scala> :require /Users/jason/.m2/repository/junit/junit/4.11/junit-4.11.jar
Added '/Users/jason/.m2/repository/junit/junit/4.11/junit-4.11.jar' to classpath.

scala> classOf[org.junit.Test]
res1: Class[org.junit.Test] = interface org.junit.Test
```

I have commited an automated test that is a minimization of this one.

2 files changed, 39 insertions, 11 deletions

diff --git a/src/repl/scala/tools/nsc/interpreter/ILoop.scala b/src/repl/scala/tools/nsc/interpreter/ILoop.scala
index 4fd5768b79..4d71e0e09e 100644
--- a/src/repl/scala/tools/nsc/interpreter/ILoop.scala
+++ b/src/repl/scala/tools/nsc/interpreter/ILoop.scala
@@ -12,6 +12,7 @@ import scala.annotation.tailrec
 import Predef.{ println => _, _ }
 import interpreter.session._
 import StdReplTags._
+import scala.tools.asm.ClassReader
 import scala.util.Properties.{ jdkHome, javaVersion, versionString, javaVmName }
 import scala.tools.nsc.util.{ ClassPath, Exceptional, stringFromWriter, stringFromStream }
 import scala.reflect.classTag
@@ -633,28 +634,29 @@ class ILoop(in0: Option[BufferedReader], protected val out: JPrintWriter)
    *  the interpreter and replays all interpreter expressions.
    */
   def require(arg: String): Unit = {
-    class InfoClassLoader extends java.lang.ClassLoader {
-      def classOf(arr: Array[Byte]): Class[_] =
-        super.defineClass(null, arr, 0, arr.length)
-    }
-
     val f = File(arg).normalize
 
-    if (f.isDirectory) {
-      echo("Adding directories to the classpath is not supported. Add a jar instead.")
+    val jarFile = AbstractFile.getDirectory(new java.io.File(arg))
+    if (jarFile == null) {
+      echo(s"Cannot load '$arg'")
       return
     }
 
-    val jarFile = AbstractFile.getDirectory(new java.io.File(arg))
-
     def flatten(f: AbstractFile): Iterator[AbstractFile] =
       if (f.isClassContainer) f.iterator.flatMap(flatten)
       else Iterator(f)
 
     val entries = flatten(jarFile)
-    val cloader = new InfoClassLoader
 
-    def classNameOf(classFile: AbstractFile): String = cloader.classOf(classFile.toByteArray).getName
+    def classNameOf(classFile: AbstractFile): String = {
+      val input = classFile.input
+      try {
+        val reader = new ClassReader(input)
+        reader.getClassName.replace('/', '.')
+      } finally {
+        input.close()
+      }
+    }
     def alreadyDefined(clsName: String) = intp.classLoader.tryToLoadClass(clsName).isDefined
     val exists = entries.filter(_.hasExtension("class")).map(classNameOf).exists(alreadyDefined)
 
diff --git a/test/files/run/t6502.scala b/test/files/run/t6502.scala
index 4ce034a482..b0d93ac3fd 100644
--- a/test/files/run/t6502.scala
+++ b/test/files/run/t6502.scala
@@ -46,6 +46,12 @@ object Test extends StoreReporterDirectTest {
       }
     }"""
 
+  def app6 = """
+    package test6
+    class A extends Test { println("created test6.A") }
+    class Z extends Test { println("created test6.Z") }
+    trait Test"""
+
   def test1(): Unit = {
     val jar = "test1.jar"
     compileCode(app1, jar)
@@ -105,11 +111,31 @@ object Test extends StoreReporterDirectTest {
     println(s"test4 res2: $res2")
   }
 
+  def test5(): Unit = {
+    val codeToRun = ":require /does/not/exist.jar"
+    val output = ILoop.run(codeToRun, settings)
+    assert(!output.contains("NullPointerException"), output)
+    assert(output.contains("Cannot load '/does/not/exist.jar'"), output)
+  }
+
+  def test6(): Unit = {
+    // Avoid java.lang.NoClassDefFoundError triggered by the old appoach of using a Java
+    // classloader to parse .class files in order to read their names.
+    val jar = "test6.jar"
+    compileCode(app6, jar)
+    val codeToRun = toCodeInSeparateLines(s":require ${testOutput.path}/$jar", "import test6._; new A; new Z")
+    val output = ILoop.run(codeToRun, settings)
+    assert(output.contains("created test6.A"), output)
+    assert(output.contains("created test6.Z"), output)
+  }
+
   def show(): Unit = {
     test1()
     test2()
     test3()
     test4()
+    test5()
+    test6()
   }
 
   def toCodeInSeparateLines(lines: String*): String = lines.map(_ + "\n").mkString