anchor.diff:    Node tree = parseStatement("if (true) { var a = 1; var b = 2; }");
     Element element = visitor.visit(tree);
     Expect.equals(null, element);
  -  BlockScope scope = visitor.scope;
  +  MethodScope scope = visitor.scope;
     Expect.equals(0, scope.elements.length);
     Expect.equals(2, map(visitor).length);
   
  
positive.code:   Node tree = parseStatement("{ var a = 1; if (true) { a; } }");
    Element element = visitor.visit(tree);
    Expect.equals(null, element);
    BlockScope scope = visitor.scope;
    Expect.equals(0, scope.elements.length);
    Expect.equals(3, map(visitor).length);
    List<Element> elements = map(visitor).getValues();
  
negative.code:   Node tree = parseStatement("{ var a = 1; if (true) { var a = 1; } }");
    Element element = visitor.visit(tree);
    Expect.equals(null, element);
    MethodScope scope = visitor.scope;
    Expect.equals(0, scope.elements.length);
    Expect.equals(2, map(visitor).length);
    List<Element> elements = map(visitor).getValues();
  