Deal with the JS helper separately from the 'native' keyword.

lib/compiler/implementation/compiler.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.
 
 
 /**
  * If true, print a warning for each method that was resolved, but not
  * compiled.
  */
 final bool REPORT_EXCESS_RESOLUTION = false;
@@ skipping 507 matching lines @@
     library.addToScope(new ForeignElement(
         const SourceString('JS_HAS_EQUALS'), library), this);
     library.addToScope(new ForeignElement(
         const SourceString('JS_CURRENT_ISOLATE'), library), this);
     library.addToScope(new ForeignElement(
         const SourceString('JS_CALL_IN_ISOLATE'), library), this);
     library.addToScope(new ForeignElement(
         const SourceString('DART_CLOSURE_TO_JS'), library), this);
   }
 
+  /** Enable the 'JS' helper for a library if needed. */
+  void maybeEnableJSHelper(library) {
+    String libraryName = library.uri.toString();
+    if (library.entryCompilationUnit.script.name.contains(
+            'dart/tests/compiler/dart2js_native')
+        || libraryName == 'dart:isolate'
+        || libraryName == 'dart:math'
+        || libraryName == 'dart:html') {
+      library.addToScope(findHelper(const SourceString('JS')), this);
+      if (jsIndexingBehaviorInterface !== null) {
+        library.addToScope(jsIndexingBehaviorInterface, this);
+      }
+    }
+  }
+
   void runCompiler(Uri uri) {
     scanBuiltinLibraries();
     mainApp = scanner.loadLibrary(uri, null, uri);
+    libraries.forEach((_, library) {
+      maybeEnableJSHelper(library);
+    });
     final Element main = mainApp.find(MAIN);
     if (main === null) {
       reportFatalError('Could not find $MAIN', mainApp);
     } else {
       if (!main.isFunction()) reportFatalError('main is not a function', main);
       FunctionElement mainMethod = main;
       FunctionSignature parameters = mainMethod.computeSignature(this);
       parameters.forEachParameter((Element parameter) {
         reportFatalError('main cannot have parameters', parameter);
       });
@@ skipping 358 matching lines @@
     final endOffset = end.charOffset + end.slowCharCount;
 
     // [begin] and [end] might be the same for the same empty token. This
     // happens for instance when scanning '$$'.
     assert(endOffset >= beginOffset);
     return f(beginOffset, endOffset);
   }
 
   String toString() => 'SourceSpan($uri, $begin, $end)';
 }