fix(@angular-devkit/build-angular): prevent differential loading double sourcemap search

This can drastically reduce memory usage; especially in cases where bundled code modules contain individual sourcemap comments and vendor sourcemaps are disabled.  Enabling the vendor sourcemap option has the side effect of removing all individual module sourcemap comments and as a result removes the potential for those comments to be found and processed.

Author: clydin

packages/angular_devkit/build_angular/src/utils/process-bundle.ts

@@ -115,15 +115,16 @@ export async function process(options: ProcessBundleOptions): Promise<ProcessBun
   const mapSize = options.map ? Buffer.byteLength(options.map) : 0;
   const manualSourceMaps = codeSize >= 500 * 1024 || mapSize >= 500 * 1024;
   const sourceCode = options.code;
-  const sourceMap = options.map ? JSON.parse(options.map) : undefined;
+  const sourceMap = options.map ? JSON.parse(options.map) : false;
 
   let downlevelCode;
   let downlevelMap;
   if (downlevel) {
     // Downlevel the bundle
     const transformResult = await transformAsync(sourceCode, {
       filename: options.filename,
-      inputSourceMap: manualSourceMaps ? undefined : sourceMap,
+      // using false ensures that babel will NOT search and process sourcemap comments (large memory usage)
+      inputSourceMap: manualSourceMaps ? false : sourceMap,
       babelrc: false,
       presets: [[
         require.resolve('@babel/preset-env'),