fix(@angular-devkit/build-angular): --vendor-source-map doesn't work on ng serve

Closes #11715

Author: alan-agius4

docs/documentation/serve.md

@@ -208,6 +208,15 @@ ng serve [project]
     Output in-file eval sourcemaps.
   </p>
 </details>
+<details>
+  <summary>vendor-source-map</summary>
+  <p>
+    <code>--vendor-source-map</code>
+  </p>
+  <p>
+    Resolve vendor packages sourcemaps.
+  </p>
+</details>
 <details>
   <summary>vendor-chunk</summary>
   <p>
@@ -261,4 +270,4 @@ ng serve [project]
   <p>
     Log progress to the console while building.
   </p>
-</details>
+</details>

packages/angular_devkit/build_angular/src/dev-server/index.ts

@@ -51,6 +51,7 @@ export interface DevServerBuilderOptions {
   optimization?: boolean;
   aot?: boolean;
   sourceMap?: boolean;
+  vendorSourceMap?: boolean;
   evalSourceMap?: boolean;
   vendorChunk?: boolean;
   commonChunk?: boolean;
@@ -399,6 +400,8 @@ export class DevServerBuilder implements Builder<DevServerBuilderOptions> {
       ...(options.optimization !== undefined ? { optimization: options.optimization } : {}),
       ...(options.aot !== undefined ? { aot: options.aot } : {}),
       ...(options.sourceMap !== undefined ? { sourceMap: options.sourceMap } : {}),
+      ...(options.vendorSourceMap !== undefined ?
+         { vendorSourceMap: options.vendorSourceMap } : {}),
       ...(options.evalSourceMap !== undefined ? { evalSourceMap: options.evalSourceMap } : {}),
       ...(options.vendorChunk !== undefined ? { vendorChunk: options.vendorChunk } : {}),
       ...(options.commonChunk !== undefined ? { commonChunk: options.commonChunk } : {}),