Only traverse send nodes in def_modifier?

A send node is a "def modifier" if it's part of a chain of method calls
preceding a method definition. However the previous implementation was
too permissive and matched any method definition in the send node's AST.

In the following example, `include` was considered to be a def modifier:

    include Module.new {
      def foo
      end
    }

CHANGELOG.md

@@ -168,3 +168,4 @@
 [@fatkodima]: https://github.com/fatkodima
 [@koic]: https://github.com/koic
 [@dvandersluis]: https://github.com/dvandersluis
+[@eugeneius]: https://github.com/eugeneius

changelog/fix_only_traverse_send_nodes_in_def_modifier.md

@@ -0,0 +1 @@
+* [#142](https://github.com/rubocop-hq/rubocop-ast/pull/142): Only traverse send nodes in `MethodDispatchNode#def_modifier?`. ([@eugeneius][])

lib/rubocop/ast/node/mixin/method_dispatch_node.rb

@@ -174,7 +174,7 @@ def arithmetic_operation?
       # @return [Boolean] whether the dispatched method is a `def` modifier
       def def_modifier?
         send_type? &&
-          [self, *each_descendant(:send)].any?(&:adjacent_def_modifier?)
+          adjacent_def_modifier? || each_child_node(:send).any?(&:def_modifier?)
       end
 
       # Checks whether this is a lambda. Some versions of parser parses

spec/rubocop/ast/send_node_spec.rb

@@ -1099,10 +1099,16 @@ def bar
     end
 
     context 'with several prefixed def modifiers' do
-      let(:source) { 'foo bar def baz; end' }
+      let(:source) { 'foo bar baz def qux; end' }
 
       it { expect(send_node).to be_def_modifier }
     end
+
+    context 'with a block containing a method definition' do
+      let(:source) { 'foo bar { baz def qux; end }' }
+
+      it { expect(send_node).not_to be_def_modifier }
+    end
   end
 
   describe '#negation_method?' do