Closes SI-6952: add correct error positions for Dynamic feature check.

src/compiler/scala/tools/nsc/typechecker/Typers.scala

@@ -1740,8 +1740,8 @@ trait Typers extends Modes with Adaptations with Tags {
      */
     def validateParentClasses(parents: List[Tree], selfType: Type) {
       val pending = ListBuffer[AbsTypeError]()
-      def validateDynamicParent(parent: Symbol) =
-        if (parent == DynamicClass) checkFeature(parent.pos, DynamicsFeature)
+      def validateDynamicParent(parent: Symbol, parentPos: Position) =
+        if (parent == DynamicClass) checkFeature(parentPos, DynamicsFeature)
 
       def validateParentClass(parent: Tree, superclazz: Symbol) =
         if (!parent.isErrorTyped) {
@@ -1791,7 +1791,7 @@ trait Typers extends Modes with Adaptations with Tags {
           if (parents exists (p => p != parent && p.tpe.typeSymbol == psym && !psym.isError))
             pending += ParentInheritedTwiceError(parent, psym)
 
-          validateDynamicParent(psym)
+          validateDynamicParent(psym, parent.pos)
         }
 
       if (!parents.isEmpty && parents.forall(!_.isErrorTyped)) {

test/files/neg/t6040.check

@@ -1,7 +1,9 @@
-error: extension of type scala.Dynamic needs to be enabled
+t6040.scala:1: error: extension of type scala.Dynamic needs to be enabled
 by making the implicit value language.dynamics visible.
 This can be achieved by adding the import clause 'import scala.language.dynamics'
 or by setting the compiler option -language:dynamics.
 See the Scala docs for value scala.language.dynamics for a discussion
 why the feature needs to be explicitly enabled.
+class X extends Dynamic
+                ^
 one error found

test/files/neg/t6952.check

@@ -0,0 +1,13 @@
+t6952.scala:2: error: extension of type scala.Dynamic needs to be enabled
+by making the implicit value language.dynamics visible.
+This can be achieved by adding the import clause 'import scala.language.dynamics'
+or by setting the compiler option -language:dynamics.
+See the Scala docs for value scala.language.dynamics for a discussion
+why the feature needs to be explicitly enabled.
+trait B extends Dynamic
+                ^
+t6952.scala:3: error: extension of type scala.Dynamic needs to be enabled
+by making the implicit value language.dynamics visible.
+trait C extends A with Dynamic
+                       ^
+two errors found

test/files/neg/t6952.scala

@@ -0,0 +1,4 @@
+trait A
+trait B extends Dynamic
+trait C extends A with Dynamic
+trait D extends B