fix(@angular/cli): add a flag to let assets outside of outDir

On top of #7778
Fixes #8122

Author: hansl

packages/@angular/cli/lib/config/schema.json

@@ -38,6 +38,11 @@
             "description": "Directory where app files are placed.",
             "default": "app"
           },
+          "allowAssetsOutsideOutDir": {
+            "type": "boolean",
+            "description": "Allow assets to be copied outside the outDir.",
+            "default": false
+          }
           "root": {
             "type": "string",
             "description": "The root directory of the app."

packages/@angular/cli/models/webpack-configs/common.ts

@@ -95,12 +95,20 @@ export function getCommonConfig(wco: WebpackConfigOptions) {
       asset.output = asset.output || '';
       asset.glob = asset.glob || '';
 
-      // Prevent asset configurations from writing outside of the output path
+      // Prevent asset configurations from writing outside of the output path, except if the user
+      // specify a configuration flag.
+      // Also prevent writing outside the project path. That is not overridable.
       const fullOutputPath = path.resolve(buildOptions.outputPath, asset.output);
-      if (!fullOutputPath.startsWith(path.resolve(buildOptions.outputPath))) {
-        const message = 'An asset cannot be written to a location outside of the output path.';
+      if (!fullOutputPath.startsWith(projectRoot)) {
+        const message = 'An asset cannot be written to a location outside the project.';
         throw new SilentError(message);
       }
+      if (!fullOutputPath.startsWith(path.resolve(buildOptions.outputPath))) {
+        if (!appConfig.allowAssetsOutsideOutDir) {
+          const message = 'An asset cannot be written to a location outside of the output path.';
+          throw new SilentError(message);
+        }
+      }
 
       // Ensure trailing slash.
       if (isDirectory(path.resolve(asset.input))) {