JavaScript: Patch CFG to improve support for non-top level import declarations.

javascript/ql/src/semmle/javascript/CFG.qll

@@ -281,7 +281,7 @@ import javascript
  */
 class ControlFlowNode extends @cfg_node, Locatable {
   /** Gets a node succeeding this node in the CFG. */
-  ControlFlowNode getASuccessor() {
+  cached ControlFlowNode getASuccessor() {
     successor(this, result)
   }
 
@@ -457,3 +457,47 @@ class ConcreteControlFlowNode extends ControlFlowNode {
     not this instanceof SyntheticControlFlowNode
   }
 }
+
+/**
+ * A CFG node corresponding to a nested (that is, non-toplevel) import declaration.
+ *
+ * This is a non-standard language feature that is not currently supported very well
+ * by the extractor, in particular such imports do not appear in the control flow graph
+ * generated by the extractor. We patch them in by overriding `getASuccessor`; once an
+ * extractor fix becomes available, this class can be removed.
+ */
+private class NestedImportDeclaration extends ImportDeclaration {
+  NestedImportDeclaration() {
+    exists (ASTNode p | p = getParent() |
+      not p instanceof TopLevel
+    ) and
+    // if there are no specifiers, the default control flow graph is fine
+    exists(getASpecifier())
+  }
+
+  override ControlFlowNode getASuccessor() {
+    result = getSpecifier(0).getFirstControlFlowNode()
+  }
+}
+
+/**
+ * A CFG node corresponding to an import specifier in a nested import declaration.
+ *
+ * As for `NestedImportDeclaration` above, this is a temporary workaround that will be
+ * removed once extractor support for this non-standard language feature becomes available.
+ */
+private class NestedImportSpecifier extends ImportSpecifier {
+  NestedImportDeclaration decl;
+  int i;
+
+  NestedImportSpecifier() {
+    this = decl.getSpecifier(i)
+  }
+
+  override ControlFlowNode getASuccessor() {
+    result = decl.getSpecifier(i+1).getFirstControlFlowNode()
+    or
+    not exists(decl.getSpecifier(i+1)) and
+    successor(decl, result)
+  }
+}