Fix a subtle bug. Since block methods can be defined in different cla…

…sses depending on the count of their arguments we must take this into consideration for guarding dispatch chains depending on blocks

src/som/interpreter/nodes/dispatch/DispatchGuard.java

@@ -22,6 +22,10 @@ public static DispatchGuard create(final Object obj) {
     if (obj instanceof DynamicObject) {
       return new CheckSObject(((DynamicObject) obj).getShape());
     }
+
+    if (obj instanceof SBlock) {
+      return new CheckSBlock(((SBlock) obj).getSOMClass());
+    }
 
     return new CheckClass(obj.getClass());
   }
@@ -88,4 +92,19 @@ public boolean entryMatches(final Object obj) throws InvalidAssumptionException
         ((DynamicObject) obj).getShape() == expected;
     }
   }
+
+  private static final class CheckSBlock extends DispatchGuard {
+    private final DynamicObject expected;
+
+    public CheckSBlock(final DynamicObject blockClass) {
+      this.expected = blockClass;
+    }
+
+  @Override
+  public boolean entryMatches(final Object obj) throws InvalidAssumptionException {
+
+    return obj instanceof SBlock &&
+        ((SBlock) obj).getSOMClass() == expected;
+    }
+  }
 }