Stop importing js_helper.dart explicitly in the core library.

lib/compiler/implementation/apiimpl.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('leg_apiimpl');
 
 #import('dart:uri');
 
 #import('../compiler.dart', prefix: 'api');
 #import('leg.dart', prefix: 'leg');
@@ skipping 30 matching lines @@
   String lookupLibraryPath(String dartLibraryName) {
     LibraryInfo info = LIBRARIES[dartLibraryName];
     if (info === null) return null;
     if (!info.isDart2jsLibrary) return null;
     String path = info.dart2jsPath;
     if (path === null) {
       path = info.path;
     }
     return "lib/$path";
   }
+
   String lookupPatchPath(String dartLibraryName) {
     LibraryInfo info = LIBRARIES[dartLibraryName];
     if (info === null) return null;
     if (!info.isDart2jsLibrary) return null;
     String path = info.dart2jsPatchPath;
     if (path === null) return null;
     return "lib/$path";
   }
 
   elements.LibraryElement scanBuiltinLibrary(String path) {
     Uri uri = libraryRoot.resolve(lookupLibraryPath(path));
-    Uri canonicalUri;
-    if (path.startsWith("_")) {
-      canonicalUri = uri;
-    } else {
+    Uri canonicalUri = null;
+    if (!path.startsWith("_")) {
       canonicalUri = new Uri.fromComponents(scheme: "dart", path: path);
     }
     elements.LibraryElement library =
         scanner.loadLibrary(uri, null, canonicalUri);
     return library;
   }
 
   void log(message) {
     handler(null, null, null, message, api.Diagnostic.VERBOSE_INFO);
   }
@@ skipping 69 matching lines @@
       handler(translateUri(span.uri, null), span.begin, span.end,
               message, kind);
     }
   }
 
   bool get isMockCompilation {
     return mockableLibraryUsed
       && (options.indexOf('--allow-mock-compilation') !== -1);
   }
 }