[frontend_server] Avoid passing package config

to DDC when the front end server was started with the
`--debugger-module-names` flag.

Followup to https://dart-review.googlesource.com/c/sdk/+/456500

It appears the flag is entirely unused in our supported environments.
There are tests for it so this change preserves the previous behavior
in the tests until I can confirm there are no real usages.

See: https://github.com/dart-lang/webdev/blob/382ff428a8bc2e32830aa9d763b1887759435ed6/dwds/test/frontend_server_circular_evaluate_test.dart#L34

Change-Id: I25688e8b0d7d69779f2d58466a0c9d076a670b61
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/459661
Commit-Queue: Nicholas Shahan <nshahan@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>

Author: nshahan

pkg/front_end/test/spell_checking_list_common.txt

@@ -2827,6 +2827,7 @@ serialized
 serializes
 serializing
 serious
+served
 set
 sets
 setter

pkg/frontend_server/lib/frontend_server.dart

@@ -214,7 +214,10 @@ ArgParser argParser = new ArgParser(allowTrailingOptions: true)
   ..addOption('libraries-spec',
       help: 'A path or uri to the libraries specification JSON file')
   ..addFlag('debugger-module-names',
-      help: 'Use debugger-friendly modules names', defaultsTo: false)
+      help: "Use debugger-friendly modules names that assume the 'lib/' "
+          "directories of packages are present in the served directory "
+          "structure at runtime.",
+      defaultsTo: false)
   ..addFlag('experimental-emit-debug-metadata',
       help: 'Emit module and library metadata for the debugger',
       defaultsTo: false)

pkg/frontend_server/lib/src/javascript_bundle.dart

@@ -351,7 +351,9 @@ class IncrementalJavaScriptBundler {
         customScheme: _fileSystemScheme,
         compiler: compiler,
         component: summaryComponent,
-        packageConfig: packageConfig,
+        // Only pass a package configuration if the serve time package paths are
+        // not being enforced by the `useDebuggerModuleNames` option.
+        packageConfig: useDebuggerModuleNames ? null : packageConfig,
       );
       final Uint8List codeBytes = utf8.encode(code.code);
       final Uint8List sourceMapBytes = utf8.encode(json.encode(code.sourceMap));