Stop importing js_helper.dart explicitly in the core library.

lib/compiler/implementation/scanner/scanner_task.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.
 
 class ScannerTask extends CompilerTask {
   ScannerTask(Compiler compiler) : super(compiler);
   String get name => 'Scanner';
 
   void scanLibrary(LibraryElement library) {
     var compilationUnit = library.entryCompilationUnit;
@@ skipping 100 matching lines @@
   }
 
   void scanElements(CompilationUnitElement compilationUnit) {
     Script script = compilationUnit.script;
     Token tokens = new StringScanner(script.text).tokenize();
     compiler.dietParser.dietParse(compilationUnit, tokens);
   }
 
   LibraryElement loadLibrary(Uri uri, Node node, Uri canonicalUri) {
     bool newLibrary = false;
-    LibraryElement library =
-      compiler.libraries.putIfAbsent(uri.toString(), () {
-          newLibrary = true;
-          Script script = compiler.readScript(uri, node);
-          LibraryElement element = new LibraryElement(script, canonicalUri);
-          native.maybeEnableNative(compiler, element, uri);
-          return element;
-        });
+    LibraryElement createLibrary() {
+      newLibrary = true;
+      Script script = compiler.readScript(uri, node);
+      LibraryElement element = new LibraryElement(script, canonicalUri);
+      native.maybeEnableNative(compiler, element, uri);
+      return element;
+    }
+    LibraryElement library;
+    if (canonicalUri === null) {
+      library = createLibrary();
+    } else {
+      library = compiler.libraries.putIfAbsent(canonicalUri.toString(),
+                                               createLibrary);
+    }
     if (newLibrary) {
       compiler.withCurrentElement(library, () {
         scanLibrary(library);
         compiler.onLibraryLoaded(library, uri);
       });
     }
     return library;
   }
 
   void importLibrary(LibraryElement library, LibraryElement imported,
@@ skipping 73 matching lines @@
   static const int RESOURCE = 4;
 
   /** Next state. */
   static const List<int> NEXT =
       const <int>[NO_TAG_SEEN,
                   IMPORT, // Only one library tag is allowed.
                   IMPORT,
                   SOURCE,
                   RESOURCE];
 }