Show import chains for unsupported dart:* library imports.

pkg/compiler/lib/src/js_backend/backend.dart

 // Copyright (c) 2012, 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.
 
 part of js_backend;
 
 const VERBOSE_OPTIMIZER_HINTS = false;
 
 class JavaScriptItemCompilationContext extends ItemCompilationContext {
   final Set<HInstruction> boundsChecked = new Set<HInstruction>();
@@ skipping 2059 matching lines @@
     closureClass = lookupHelperClass('Closure');
     if (!missingHelperClasses.isEmpty) {
       compiler.internalError(jsHelperLibrary,
           'dart:_js_helper library does not contain required classes: '
           '$missingHelperClasses');
     }
   }
 
   Future onLibraryScanned(LibraryElement library, LibraryLoader loader) {
     return super.onLibraryScanned(library, loader).then((_) {
-      if (library.isPlatformLibrary && !library.isPatched) {
+      if (library.isPlatformLibrary &&
+          // Don't patch library currently disallowed.
+          !library.isSynthesized &&
+          !library.isPatched) {
         // Apply patch, if any.
         Uri patchUri = compiler.resolvePatchUri(library.canonicalUri.path);
         if (patchUri != null) {
           return compiler.patchParser.patchLibrary(loader, patchUri, library);
         }
       }
     }).then((_) {
       Uri uri = library.canonicalUri;
 
       FunctionElement findMethod(String name) {
@@ skipping 1066 matching lines @@
   }
 }
 
 /// Records that [constant] is used by the element behind [registry].
 class Dependency {
   final ConstantValue constant;
   final Element annotatedElement;
 
   const Dependency(this.constant, this.annotatedElement);
 }