feat(bazel): allow passing and rewriting an old bazel host

Updates the decision made in angular#31341; this is for the Angular indexer
inside Google. The indexer currently passes (and ngc-wrapped#compile
accepts) a bazel host to use, but because many methods are overwritten
specially for Angular compilation a better approach is to pass an old
bazel compiler host and siphon methods needed off of it before creating
a new host. This enables that.

packages/bazel/src/ngc-wrapped/index.ts

@@ -145,15 +145,15 @@ export function relativeToRootDirs(filePath: string, rootDirs: string[]): string
 }
 
 export function compile({allDepsCompiledWithBazel = true, compilerOpts, tsHost, bazelOpts, files,
-                         inputs, expectedOuts, gatherDiagnostics, bazelHost}: {
+                         inputs, expectedOuts, gatherDiagnostics, oldBazelHost}: {
   allDepsCompiledWithBazel?: boolean,
   compilerOpts: ng.CompilerOptions,
   tsHost: ts.CompilerHost, inputs?: {[path: string]: string},
   bazelOpts: BazelOptions,
   files: string[],
   expectedOuts: string[],
   gatherDiagnostics?: (program: ng.Program) => ng.Diagnostics,
-  bazelHost?: CompilerHost,
+  oldBazelHost?: CompilerHost,
 }): {diagnostics: ng.Diagnostics, program: ng.Program} {
   let fileLoader: FileLoader;
 
@@ -246,9 +246,10 @@ export function compile({allDepsCompiledWithBazel = true, compilerOpts, tsHost,
         moduleName, containingFile, compilerOptions, generatedFileModuleResolverHost);
   }
 
-  if (!bazelHost) {
-    bazelHost = new CompilerHost(
-        files, compilerOpts, bazelOpts, tsHost, fileLoader, generatedFileModuleResolver);
+  const bazelHost = new CompilerHost(
+      files, compilerOpts, bazelOpts, tsHost, fileLoader, generatedFileModuleResolver);
+  if (oldBazelHost) {
+    Object.assign(bazelHost, oldBazelHost);
   }
 
   // Also need to disable decorator downleveling in the BazelHost in Ivy mode.