Follow up improvement for checking dart:_builtin

pkg/front_end/lib/src/fasta/kernel/body_builder.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.body_builder;
 
 import '../fasta_codes.dart'
     show FastaMessage, codeExpectedButGot, codeExpectedFunctionBody;
 
 import '../parser/parser.dart' show FormalParameterType, MemberKind, optional;
@@ skipping 66 matching lines @@
 
   final bool isInstanceMember;
 
   final Map<String, FieldInitializer> fieldInitializers =
       <String, FieldInitializer>{};
 
   final Scope enclosingScope;
 
   final bool enableNative;
 
-  final bool isPlatformLibrary;
+  final bool isBuiltinLibrary;
 
   @override
   final Uri uri;
 
   final TypeInferrer _typeInferrer;
 
   @override
   final TypePromoter<Expression, VariableDeclaration> typePromoter;
 
   /// Only used when [member] is a constructor. It tracks if an implicit super
@@ skipping 41 matching lines @@
       this.formalParameterScope,
       this.hierarchy,
       this.coreTypes,
       this.classBuilder,
       this.isInstanceMember,
       this.uri,
       this._typeInferrer)
       : enclosingScope = scope,
         library = library,
         enableNative = library.loader.target.enableNative(library),
-        isPlatformLibrary = library.uri.scheme == 'dart',
+        isBuiltinLibrary =
+            library.uri.scheme == 'dart' && library.uri.path == "_builtin",
         needsImplicitSuperInitializer =
             coreTypes.objectClass != classBuilder?.cls,
         typePromoter = _typeInferrer.typePromoter,
         super(scope);
 
   bool get hasParserError => recoverableErrors.isNotEmpty;
 
   bool get inConstructor {
     return functionNestingLevel == 0 && member is KernelConstructorBuilder;
   }
@@ skipping 737 matching lines @@
           builder == null &&
           "loadLibrary" == name) {
         return buildCompileTimeError(
             "Deferred loading isn't implemented yet.", offsetForToken(token));
       } else if (!isQualified && isInstanceContext) {
         assert(builder == null);
         if (constantExpressionRequired) {
           return new UnresolvedAccessor(this, n, token);
         }
         return new ThisPropertyAccessor(this, token, n, null, null);
-      } else if (
-          // Optimization, if [isPlatformLibrary] is false, this can't be
-          // dart:_builtin.
-          isPlatformLibrary &&
-              name == "main" &&
-              library.uri.path == "_builtin" &&
-              member?.name == "_getMainClosure") {
+      } else if (isBuiltinLibrary &&
+          name == "main" &&
+          member?.name == "_getMainClosure") {
         // TODO(ahe): https://github.com/dart-lang/sdk/issues/28989
         return new KernelNullLiteral()..fileOffset = offsetForToken(token);
       } else {
         return new UnresolvedAccessor(this, n, token);
       }
     } else if (builder.isTypeDeclaration) {
       if (constantExpressionRequired &&
           builder.isTypeVariable &&
           !member.isConstructor) {
         addCompileTimeError(
@@ skipping 2351 matching lines @@
     if (starToken == null) {
       return AsyncMarker.Async;
     } else {
       assert(identical(starToken.stringValue, "*"));
       return AsyncMarker.AsyncStar;
     }
   } else {
     return internalError("Unknown async modifier: $asyncToken");
   }
 }