Stop importing js_helper.dart explicitly in the core library.

tests/compiler/dart2js/unparser_test.dart

Index: tests/compiler/dart2js/unparser_test.dart
diff --git a/tests/compiler/dart2js/unparser_test.dart b/tests/compiler/dart2js/unparser_test.dart
index ffe478ce6d5fff0e639af4ae847218bc943d6008..89feb197e36736a29c4e909789a603044e897e33 100644
--- a/tests/compiler/dart2js/unparser_test.dart
+++ b/tests/compiler/dart2js/unparser_test.dart
@@ -48,6 +48,11 @@ class Platform {
 }
 ''';
 
+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.
@@ -76,6 +81,9 @@ testDart2DartWithLibrary(
     }
     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('');
   }