code: function configure(pkg, env, target) {
      const isModule = target === 'module';
      const input = `src/index.js`;
      const deps = []
          .concat(pkg.dependencies ? Object.keys(pkg.dependencies) : [])
          .concat(pkg.peerDependencies ? Object.keys(pkg.peerDependencies) : []);
  
      const plugins = [
          // Allow Rollup to resolve modules from `node_modules`, since it only
          // resolves local modules by default.
          resolve({
              browser: true
          }),
  
          // Convert JSON imports to ES6 modules.
          json(),
  
          // Replace `process.env.NODE_ENV` with its value, which enables some modules
          // like React and Slate to use their production variant.
          replace({
              'process.env.NODE_ENV': JSON.stringify(env)
          }),
  
          // Register Node.js builtins for browserify compatibility.
          builtins(),
  
          // Use Babel to transpile the result, limiting it to the source code.
          babel({
              include: [`src/**`]
          }),
  
          // Register Node.js globals for browserify compatibility.
          globals()
      ].filter(Boolean);
  
      if (isModule) {
          return {
              plugins,
              input,
              output: [
                  {
                      file: `${pkg.module}`,
                      format: 'es',
                      sourcemap: true
                  },
                  {
                      file: `${pkg.main}`,
                      format: 'cjs',
                      exports: 'named',
                      sourcemap: true
                  }
              ],
              // We need to explicitly state which modules are external, meaning that
              // they are present at runtime. In the case of non-UMD configs, this means
              // all non-Slate packages.
              external: id =>
                  !!deps.find(dep => dep === id || id.startsWith(`${dep}/`))
          };
      }
      return undefined;
  }
docs: Return a Rollup configuration for a `pkg` with `env` and `target`.
  
  @param {Object} pkg
  @param {String} env
  @param {String} format
  @return {Object}