code: function findAllExpressions(parentBlockStatement, expn, text) {
          var doc   = session.editor.document,
              obj   = {},
              expns = [];
  
          // find all references of the expression
          obj[expn.type] = function(node) {
              if (text === doc.getText().substr(node.start, node.end - node.start)) {
                  expns.push(node);
              }
          };
          ASTWalker.simple(parentBlockStatement, obj);
  
          return expns;
      }
docs: Find all expressions in the parentBlockStatement that are same as expn
  @param {!ASTNode} parentBlockStatement
  @param {!ASTNode} expn
  @param {!string} text - text of the expression
  @return {!Array.<ASTNode>}