Fix performance regression for checking that retricted types are not

pkg/compiler/lib/src/kernel/fasta_support.dart

Index: pkg/compiler/lib/src/kernel/fasta_support.dart
diff --git a/pkg/compiler/lib/src/kernel/fasta_support.dart b/pkg/compiler/lib/src/kernel/fasta_support.dart
index af3b833499166340dc5fbbff5f0f6be69122e314..b96e840f97a909288fd4b45c7fa320c64863c3af 100644
--- a/pkg/compiler/lib/src/kernel/fasta_support.dart
+++ b/pkg/compiler/lib/src/kernel/fasta_support.dart
@@ -18,10 +18,12 @@ library compiler.src.kernel.fasta_support;
 import 'dart:async' show Future;
 import 'dart:io' show exitCode;
 
+import 'package:front_end/file_system.dart';
 import 'package:front_end/physical_file_system.dart';
 import 'package:front_end/src/fasta/kernel/utils.dart';
-import 'package:kernel/ast.dart' show Source;
+import 'package:kernel/ast.dart' show Source, Library;
 
+import 'package:front_end/src/fasta/builder/builder.dart' show LibraryBuilder;
 import 'package:front_end/src/fasta/compiler_context.dart' show CompilerContext;
 import 'package:front_end/src/fasta/dill/dill_target.dart' show DillTarget;
 import 'package:front_end/src/fasta/fasta.dart' show CompileTask;
@@ -29,6 +31,8 @@ import 'package:front_end/src/fasta/kernel/kernel_target.dart'
     show KernelTarget;
 import 'package:front_end/src/fasta/loader.dart' show Loader;
 import 'package:front_end/src/fasta/parser/parser.dart' show optional;
+import 'package:front_end/src/fasta/source/source_loader.dart'
+    show SourceLoader;
 import 'package:front_end/src/scanner/token.dart' show Token;
 import 'package:front_end/src/fasta/ticker.dart' show Ticker;
 import 'package:front_end/src/fasta/translate_uri.dart' show TranslateUri;
@@ -87,6 +91,10 @@ class KernelTargetForDart2js extends KernelTarget {
             uriToSource);
 
   @override
+  SourceLoader<Library> createLoader() =>
+      new SourceLoaderForDart2js<Library>(fileSystem, this);
+
+  @override
   Token skipNativeClause(Token token) => _skipNative(token);
 
   @override
@@ -99,6 +107,31 @@ class KernelTargetForDart2js extends KernelTarget {
   void runBuildTransformations() {}
 }
 
+/// Specializes [SourceLoader] to build kernel for dart2js: dart2js extends
+/// bool, int, num, double, and String in a different platform library than
+/// `dart:core`.
+class SourceLoaderForDart2js<L> extends SourceLoader<L> {
+  LibraryBuilder interceptorsLibrary;
+
+  @override
+  LibraryBuilder read(Uri uri, int charOffset,
+      {Uri fileUri, LibraryBuilder accessor, bool isPatch: false}) {
+    var library = super.read(uri, charOffset,
+        fileUri: fileUri, accessor: accessor, isPatch: isPatch);
+    if (uri.scheme == 'dart' && uri.path == '_interceptors') {
+      interceptorsLibrary = library;
+    }
+    return library;
+  }
+
+  @override
+  bool canImplementRestrictedTypes(LibraryBuilder library) =>
+      library == coreLibrary || library == interceptorsLibrary;
+
+  SourceLoaderForDart2js(FileSystem fs, KernelTarget target)
+      : super(fs, target);
+}
+
 /// Specializes [DillTarget] to build kernel for dart2js: JS-specific libraries
 /// are included in the SDK, and native clauses have no string parameter.
 class DillTargetForDart2js extends DillTarget {