Add support for parsing system classes.

compile/api/src/main/scala/sbt/ClassToAPI.scala

@@ -160,7 +160,7 @@ object ClassToAPI {
           } catch {
             case e: Throwable =>
               throw new IllegalStateException(
-                s"Failed to parse class $c: this may mean your classfiles are corrupted. Please clean and try again.",
+                s"Failed to parse $c: this may mean your classfiles are corrupted. Please clean and try again.",
                 e
               )
           }

util/io/src/main/scala/sbt/IO.scala

@@ -38,14 +38,22 @@ object IO {
   /**
    * Returns a URL for the directory or jar containing the the class file `cl`.
    * If the location cannot be determined, an error is generated.
-   * Note that Java standard library classes typically do not have a location associated with them.
    */
-  def classLocation(cl: Class[_]): URL =
+  def classLocation(cl: Class[_]): URL = {
-    {
+    val codeSource = cl.getProtectionDomain.getCodeSource
-      val codeSource = cl.getProtectionDomain.getCodeSource
+    if (codeSource ne null) {
-      if (codeSource == null) sys.error("No class location for " + cl)
+      codeSource.getLocation
-      else codeSource.getLocation
+    } else {
+      Option(ClassLoader.getSystemClassLoader.getResource(classfilePathForClassname(cl.getName)))
+        .flatMap {
+          // TODO: assuming that System-class-loaded classes are located in jars, which
+          // will cause this method to truncate to the jar
+          urlAsFile
+        }.getOrElse {
+          sys.error("No class location for " + cl)
+        }.toURI.toURL
     }
+  }
 
   /**
    * Returns the directory or jar file containing the the class file `cl`.
@@ -88,6 +96,12 @@ object IO {
       case _ => None
     }
 
+  /**
+   * @return The path for the given classname.
+   *
+   * TODO: crossplatform
+   */
+  def classfilePathForClassname(clsname: String): String = s"${clsname.replace('.', '/')}.class"
   private[this] def uriToFile(uriString: String): File =
     {
       val uri = new URI(uriString)