refactor(compiler): Reintroduce ReflectorHost and move Extractor into @angular/compiler

modules/@angular/compiler/src/output/path_util.ts

@@ -10,5 +10,9 @@
  * Interface that defines how import statements should be generated.
  */
 export abstract class ImportResolver {
+  /**
+   * Converts a file path to a module name that can be used as an `import.
+   * I.e. `path/to/importedFile.ts` should be imported by `path/to/containingFile.ts`.
+   */
   abstract fileNameToModuleName(importedFilePath: string, containingFilePath: string): string;
 }