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.

1 files changed, 26 insertions, 0 deletions

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