Load builtin helper libraries on demand.

sdk/lib/_internal/compiler/implementation/native_handler.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.
 
 library native;
 
 import 'dart:collection' show Queue;
 import 'dart2jslib.dart' hide SourceString;
 import 'dart_types.dart';
 import 'elements/elements.dart';
@@ skipping 101 matching lines @@
 
   ClassElement _annotationCreatesClass;
   ClassElement _annotationReturnsClass;
   ClassElement _annotationJsNameClass;
 
   /// Subclasses of [NativeEnqueuerBase] are constructed by the backend.
   NativeEnqueuerBase(this.world, this.compiler, this.enableLiveTypeAnalysis);
 
   void processNativeClasses(Iterable<LibraryElement> libraries) {
     libraries.forEach(processNativeClassesInLibrary);
-    processNativeClassesInLibrary(compiler.isolateHelperLibrary);
-
+    if (compiler.isolateHelperLibrary != null) {
+      processNativeClassesInLibrary(compiler.isolateHelperLibrary);
+    }
     processSubclassesOfNativeClasses(libraries);
-
     if (!enableLiveTypeAnalysis) {
       nativeClasses.forEach((c) => enqueueClass(c, 'forced'));
       flushQueue();
     }
   }
 
   void processNativeClassesInLibrary(LibraryElement library) {
     // Use implementation to ensure the inclusion of injected members.
     library.implementation.forEachLocalMember((Element element) {
       if (element.isClass() && element.isNative()) {
@@ skipping 1007 matching lines @@
           'native "..." syntax is restricted to functions with zero parameters',
           node: nativeBody);
     }
     LiteralString jsCode = nativeBody.asLiteralString();
     builder.push(new HForeign.statement(
         new js.LiteralStatement(jsCode.dartString.slowToString()),
         <HInstruction>[],
         new SideEffects()));
   }
 }