Add Bytecode test (ASM-based) to partest.

This commit introduces a new kind of test `Bytecode` that allows
one to inspect bytecode generated for given piece of Scala code.

The bytecode inspection is achieved by inspection of ASM trees.
See the included example for details.

NOTE: This commit does not introduce a new category of pratest tests.
Bytecode tests should be run in `jvm` category of partest tests.

Specific list of changes:
  * Add BytecodeTest that contains common utilities to partest
  * Add asm to classpath when compiling partest. That's not a
    new dependency as it's being already done for javac task
    we were running while compiling partest.
  * Add an example test that shows how to count null checks in
    given method.

3 files changed, 42 insertions, 0 deletions

diff --git a/test/files/jvm/bytecode-test-example.check b/test/files/jvm/bytecode-test-example.check
new file mode 100644
index 0000000000..0cfbf08886
--- /dev/null
+++ b/test/files/jvm/bytecode-test-example.check
@@ -0,0 +1 @@
+2
diff --git a/test/files/jvm/bytecode-test-example/Foo_1.scala b/test/files/jvm/bytecode-test-example/Foo_1.scala
new file mode 100644
index 0000000000..4f679d156f
--- /dev/null
+++ b/test/files/jvm/bytecode-test-example/Foo_1.scala
@@ -0,0 +1,9 @@
+class Foo_1 {
+  def foo(x: AnyRef): Int = {
+    val bool = x == null
+    if (x != null)
+      1
+    else
+      0
+  }
+}
diff --git a/test/files/jvm/bytecode-test-example/Test.scala b/test/files/jvm/bytecode-test-example/Test.scala
new file mode 100644
index 0000000000..d668059cb7
--- /dev/null
+++ b/test/files/jvm/bytecode-test-example/Test.scala
@@ -0,0 +1,32 @@
+import scala.tools.partest.BytecodeTest
+
+import scala.tools.nsc.util.JavaClassPath
+import java.io.InputStream
+import scala.tools.asm
+import asm.ClassReader
+import asm.tree.{ClassNode, InsnList}
+import scala.collection.JavaConverters._
+
+object Test extends BytecodeTest {
+  def show: Unit = {
+    val classNode = loadClassNode("Foo_1")
+    val methodNode = getMethod(classNode, "foo")
+    println(countNullChecks(methodNode.instructions))
+  }
+
+  def countNullChecks(insnList: InsnList): Int = {
+    /** Is given instruction a null check?
+     *  NOTE
+     *   This will detect direct null compparsion as in
+     *    if (x == null) ...
+     *   and not indirect as in
+     *     val foo = null
+     *     if (x == foo) ...
+     */
+    def isNullCheck(node: asm.tree.AbstractInsnNode): Boolean = {
+      val opcode = node.getOpcode
+      (opcode == asm.Opcodes.IFNULL) || (opcode == asm.Opcodes.IFNONNULL)
+    }
+    insnList.iterator.asScala.count(isNullCheck)
+  }
+}