Avoid dynamic findFrameSlot

- this gave an error with the Graal optimizer, because it wasn't clear that the slot can't be null.
[TODO-REPORT: FrameDescriptor.findFrameSlot should not be used in fast-path code, try to standardize frame layout]

Signed-off-by: Stefan Marr <git@stefan-marr.de>

Author: smarr

src/som/interpreter/nodes/ContextualNode.java

@@ -21,6 +21,7 @@
  */
 package som.interpreter.nodes;
 
+import som.compiler.MethodGenerationContext;
 import som.vmobjects.Block;
 
 import com.oracle.truffle.api.frame.FrameSlot;
@@ -42,7 +43,7 @@ protected MaterializedFrame determineContext(MaterializedFrame frame) {
 
       while (i > 0) {
         try {
-          FrameSlot blockSelfSlot = ctx.getFrameDescriptor().findFrameSlot("self");
+          FrameSlot blockSelfSlot = MethodGenerationContext.getStandardSelfSlot();
           Block block = (Block) ctx.getObject(blockSelfSlot);
           ctx = block.getContext();
           i--;