fix(ngc): add an option to produce TS1.9-pathMapping imports (#10602)

This fixes a regression in #10486

tools/@angular/tsc-wrapped/src/options.ts

@@ -18,6 +18,18 @@ interface Options extends ts.CompilerOptions {
 
   // Whether to embed debug information in the compiled templates
   debug?: boolean;
+
+  // Starting with TypeScript 1.9, the 'rootDirs' option can be used
+  // to allow multiple source directories to have relative imports
+  // between them.
+  // This option causes generated code to use imports relative to the
+  // current directory, and requires you configure the 'rootDirs' to
+  // include both the genDir and rootDir.
+  // However, due to https://github.com/Microsoft/TypeScript/issues/8245
+  // note that using this option does not lay out into a flat directory
+  // with application and generated sources side-by-side, so you must
+  // teach your module loader how to resolve such imports as well.
+  writeImportsForRootDirs?: boolean;
 }
 
 export default Options;