Stop importing js_helper.dart explicitly in the core library.

tests/compiler/dart2js/unparser_test.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.
 
 #import('dart:uri');
 #import('parser_helper.dart');
 #import('mock_compiler.dart');
 #import("../../../lib/compiler/compiler.dart");
 #import("../../../lib/compiler/implementation/leg.dart", prefix:'leg');
 #import("../../../lib/compiler/implementation/dart_backend/dart_backend.dart");
@@ skipping 30 matching lines @@
 }
 ''';
 
 const ioLib = r'''
 #library('io');
 class Platform {
   static int operatingSystem;
 }
 ''';
 
+const helperLib = r'''
+#library('_js_helper');
+void assert(x);
+''';
+
 testDart2Dart(String src, [void continuation(String s), bool minify = false,
     bool cutDeclarationTypes = false]) {
   // If continuation is not provided, check that source string remains the same.
   if (continuation === null) {
     continuation = (s) { Expect.equals(src, s); };
   }
   testDart2DartWithLibrary(src, '', continuation, minify, cutDeclarationTypes);
 }
 
 /**
  * Library name is assumed to be 'mylib' in 'mylib.dart' file.
  */
 testDart2DartWithLibrary(
     String srcMain, String srcLibrary,
     [void continuation(String s), bool minify = false,
     bool cutDeclarationTypes = false]) {
   fileUri(path) => new Uri.fromComponents(scheme: 'file', path: path);
 
   final scriptUri = fileUri('script.dart');
   final libUri = fileUri('mylib.dart');
 
   provider(uri) {
     if (uri == scriptUri) return new Future.immediate(srcMain);
     if (uri.toString() == libUri.toString()) {
       return new Future.immediate(srcLibrary);
     }
     if (uri.path.endsWith('/core.dart')) return new Future.immediate(coreLib);
     if (uri.path.endsWith('/io.dart')) return new Future.immediate(ioLib);
+    if (uri.path.endsWith('/js_helper.dart')) {
+      return new Future.immediate(helperLib);
+    }
     return new Future.immediate('');
   }
 
   handler(uri, begin, end, message, kind) {
     if (kind === Diagnostic.ERROR || kind === Diagnostic.CRASH) {
       Expect.fail('$uri: $begin-$end: $message [$kind]');
     }
   }
 
   final options = <String>['--output-type=dart'];
@@ skipping 759 matching lines @@
   testDoubleMains();
   testInterfaceDefaultAnotherLib();
   testStaticAccessIoLib();
   testLocalFunctionPlaceholder();
   testMinification();
   testClosureLocalsMinified();
   testParametersMinified();
   testTypeVariablesInDifferentLibraries();
   testDeclarationTypePlaceholders();
 }