code: function addTopDeclarationNodes(ast: Object, declarationNodes: Object[]) {
    const statements = [];
    declarationNodes.forEach(declarationNode => {
      statements.push(getDeclarations(declarationNode));
    });
    statements.push(ast);
    return t.program(statements);
  }
docs: Given an AST and an array of variableDeclaration nodes, return a new AST with
  all the declarations at the top of the AST.