Move some of analyzer's code generation utilities into front_end.

pkg/front_end/test/subpackage_relationships_test.dart

Index: pkg/front_end/test/subpackage_relationships_test.dart
diff --git a/pkg/front_end/test/subpackage_relationships_test.dart b/pkg/front_end/test/subpackage_relationships_test.dart
index 629e37ef20ca7bd12dc8de2b1c1482a6e560fa15..7cd60233db0b4fac4c1f6aaa7ba9b2b2405f1d38 100644
--- a/pkg/front_end/test/subpackage_relationships_test.dart
+++ b/pkg/front_end/test/subpackage_relationships_test.dart
@@ -31,6 +31,7 @@ final subpackageRules = {
       allowedDependencies: ['lib', 'lib/src/base', 'lib/src/scanner']),
   'lib/src/base': new SubpackageRules(
       mayImportAnalyzer: true, allowedDependencies: ['lib']),
+  'lib/src/codegen': new SubpackageRules(),
   'lib/src/fasta':
       new SubpackageRules(mayImportAnalyzer: false, allowedDependencies: [
     'lib/src/fasta/builder',