Move helpers and interceptors to own library.

dart/frog/leg/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 scan(CompilationUnitElement compilationUnit) {
     measure(() {
@@ skipping 20 matching lines @@
         }
       } else if (tag.isSource()) {
         Script script = compiler.readScript(resolved, tag);
         CompilationUnitElement unit =
           new CompilationUnitElement(script, library);
         compiler.withCurrentElement(unit, () => scan(unit));
       } else {
         compiler.cancel("illegal script tag: ${tag.tag}", node: tag);
       }
     }
-    if (library !== compiler.coreLibrary) {
+    if (compiler.coreLibrary !== null) {
       importLibrary(library, compiler.coreLibrary, null);
     }
   }
 
   void scanElements(CompilationUnitElement compilationUnit) {
     compiler.log("scanning $compilationUnit");
     Script script = compilationUnit.script;
     Token tokens;
     try {
       tokens = new StringScanner(script.text).tokenize();
@@ skipping 34 matching lines @@
     } else {
       for (Link<Element> link = imported.topLevelElements; !link.isEmpty();
            link = link.tail) {
         compiler.withCurrentElement(link.head, () {
             library.define(link.head, compiler);
           });
       }
     }
   }
 }