JavaConversions: Restore source compatibility with 2.9

Readd an implicit conversion which was available in 2.9, the one from
`java.util.concurrent.ConcurrentMap` (`juc.ConcurrentMap`) to the (now
deprecated) type `scala.collection.mutable.ConcurrentMap`.

This implicit conversion can also be used to convert from
`juc.ConcurrentMap`
to `collection.Map` and creates an ambiguity error in
test/files/run/map_java_conversions.scala. To fix this, I have given
lower priority to the new conversion.

Moreover, update the documentation in `JavaConversions`: mark this
conversion as deprecated and mention the new conversion which replaces
it, converting to `scala.collection.concurrent.Map`.

I discussed this issue previously with Paul Phillips on scala-language:

https://groups.google.com/d/topic/scala-language/uXKRiGXb-44/discussion

1 files changed, 11 insertions, 0 deletions

diff --git a/test/files/pos/javaConversions-2.10-regression.scala b/test/files/pos/javaConversions-2.10-regression.scala
new file mode 100644
index 0000000000..619c44b46d
--- /dev/null
+++ b/test/files/pos/javaConversions-2.10-regression.scala
@@ -0,0 +1,11 @@
+import collection.{JavaConversions, mutable}
+import JavaConversions._
+import java.util.concurrent.ConcurrentHashMap
+
+object Foo {
+  def buildCache2_9_simple[K <: AnyRef, V <: AnyRef]: mutable.ConcurrentMap[K, V] =
+    asScalaConcurrentMap(new ConcurrentHashMap())
+
+  def buildCache2_9_implicit[K <: AnyRef, V <: AnyRef]: mutable.ConcurrentMap[K, V] =
+    new ConcurrentHashMap[K, V]()
+}