checkExport_(tree, name) {
    let description = this.validatingModuleDescription_;
    if (description && !description.getExport(name)) {
      let moduleName = description.normalizedName;
      this.reportError(tree, `'${name}' is not exported by '${moduleName}'`);
    }
  }
Validates that symbols are exported when we extract them.

  export {a as b} from 'm'
  import {c as d} from 'n'

validates that 'm' exports a and that 'n' exports c.