code: function wrapProgram(tree, url, commonPath) {
    var name = generateNameForUrl(url, commonPath);
    return new Program(null,
        [new ModuleDefinition(null,
            createIdentifierToken(name), tree.programElements)]);
  }
docs: Wraps a program in a module definition.
  @param  {ProgramTree} tree The original program tree.
  @param  {string} url The relative URL of the module that the program
  represents.
  @param {string} commonPath The base path of all the files. This is passed
  along to |generateNameForUrl|.
  @return {[ProgramTree} A new program tree with only one statement, which is
  a module definition.