make DestructuringPattern non-abstract

Author: erik-krogh

javascript/ql/src/LanguageFeatures/NonLinearPattern.ql

@@ -14,9 +14,8 @@
 
 import javascript
 
-class RootDestructuringPattern extends BindingPattern {
+class RootDestructuringPattern extends DestructuringPattern {
   RootDestructuringPattern() {
-    this instanceof DestructuringPattern and
     not this = any(PropertyPattern p).getValuePattern() and
     not this = any(ArrayPattern p).getAnElement()
   }

javascript/ql/src/semmle/javascript/Variables.qll

@@ -406,6 +406,8 @@ class BindingPattern extends @pattern, Expr {
   }
 }
 
+private class TDestructuringPattern = @arraypattern or @objectpattern;
+
 /**
  * A destructuring pattern, that is, either an array pattern or an object pattern.
  *
@@ -418,9 +420,9 @@ class BindingPattern extends @pattern, Expr {
  * }
  * ```
  */
-abstract class DestructuringPattern extends BindingPattern {
+class DestructuringPattern extends TDestructuringPattern, BindingPattern {
   /** Gets the rest pattern of this destructuring pattern, if any. */
-  abstract Expr getRest();
+  Expr getRest() { none() } // Overridden in subtypes.
 }
 
 /**