feat(@ngtools/webpack): create compiler host for webpack child compilations

Author: filipesilva

packages/ngtools/webpack/src/angular_compiler_plugin.ts

@@ -822,6 +822,22 @@ export class AngularCompilerPlugin {
       throw new Error('An @ngtools/webpack plugin already exist for this compilation.');
     }
 
+    // If there is no compiler host at this point, it means that the environment hook did not run.
+    // This happens in child compilations that inherit the parent compilation file system.
+    if (this._compilerHost === undefined) {
+      // Create a simplified compiler host for child compilations.
+      const host: virtualFs.Host<fs.Stats> = new WebpackInputHost(
+        compilation.compiler.inputFileSystem,
+      );
+      const webpackCompilerHost = new WebpackCompilerHost(
+        this._compilerOptions, this._basePath, host, true, true,
+      );
+      this._compilerHost = createCompilerHost({
+        options: this._compilerOptions,
+        tsHost: webpackCompilerHost,
+      }) as CompilerHost & WebpackCompilerHost;
+    }
+
     // Set a private variable for this plugin instance.
     // tslint:disable-next-line:no-any
     (compilation as any)._ngToolsWebpackPluginInstance = this;