Allow to load the build on JDK 9.

We put all our build-level `.scala` files in a `package build`,
rather than in the default package, to work around
scala/scala-dev#304.

This does not yet allow to build the test suite.

Author: sjrd

build.sbt

@@ -1,3 +1,5 @@
+import build.Build
+
 val scalajs = Build.root
 val ir = Build.irProject
 val irJS = Build.irProjectJS

project/BinaryIncompatibilities.scala

@@ -1,3 +1,5 @@
+package build
+
 import com.typesafe.tools.mima.core._
 import com.typesafe.tools.mima.core.ProblemFilters._
 

project/Build.scala

@@ -1,3 +1,5 @@
+package build
+
 import sbt._
 import Keys._
 
@@ -436,16 +438,10 @@ object Build {
       ),
       // JDK version we are running with
       javaVersion in Global := {
-        val v = System.getProperty("java.version")
-        v.substring(0, 3) match {
-          case "1.8" => 8
-          case "1.7" => 7
-          case "1.6" => 6
-
-          case _ =>
-            sLog.value.warn(s"Unknown JDK version $v. Assuming max compat.")
-            Int.MaxValue
-        }
+        val fullVersion = System.getProperty("java.version")
+        val v = fullVersion.stripPrefix("1.").takeWhile(_.isDigit).toInt
+        sLog.value.info(s"Detected JDK version $v")
+        v
       }
   )
 

project/ExternalCompile.scala

@@ -1,3 +1,5 @@
+package build
+
 import sbt._
 import Keys._
 

project/JavaLangObject.scala

@@ -1,3 +1,5 @@
+package build
+
 /*
  * Hard-coded IR for java.lang.Object.
  */

project/JavaLangString.scala

@@ -1,3 +1,5 @@
+package build
+
 /*
  * Hard-coded IR for java.lang.String.
  */

project/build.sbt

@@ -38,7 +38,7 @@ sources in Compile +=
   baseDirectory.value / "project" / "ScalaJSEnvGenerator.scala"
 
 sourceGenerators in Compile += Def.task {
-  ScalaJSEnvGenerator.generateEnvHolder(
+  build.ScalaJSEnvGenerator.generateEnvHolder(
     baseDirectory.value.getParentFile / "tools",
     (sourceManaged in Compile).value)
 }.taskValue

project/project/ScalaJSEnvGenerator.scala

@@ -1,3 +1,5 @@
+package build
+
 import sbt._
 
 object ScalaJSEnvGenerator {