Make sure that libraries loaded from dart:core gets dart:core implicitly loaded.

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 60 matching lines @@
     }
 
     // Apply patch, if any.
     if (library.uri.scheme == 'dart') {
       compiler.patchDartLibrary(library, library.uri.path);
     }
 
     // Now that we have processed all the source tags, it is safe to
     // start loading other libraries.
 
-    // TODO(ahe): During Compiler.scanBuiltinLibraries,
-    // compiler.coreLibrary is null. Clean this up when there is a
-    // better way to access "dart:core".
-    bool implicitlyImportCoreLibrary = compiler.coreLibrary !== null;
+    if (library.uri.scheme != 'dart' || library.uri.path != 'core') {
+      compiler.importCoreLibrary(library);
+    }
 
     for (ScriptTag tag in imports.toLink()) {
-      Uri resolvedUri =
-          importLibraryFromTag(tag, library.entryCompilationUnit);
-      if (resolvedUri.toString() == "dart:core") {
-        implicitlyImportCoreLibrary = false;
-      }
-    }
-    if (implicitlyImportCoreLibrary) {
-      importLibrary(library, compiler.coreLibrary, null);
+      importLibraryFromTag(tag, library.entryCompilationUnit);
     }
   }
 
   /**
    * Handle an import script tag by importing the referenced library into the
    * current library.
    * Returns the resolved library [Uri].
    */
   Uri importLibraryFromTag(ScriptTag tag,
                            CompilationUnitElement compilationUnit) {
@@ skipping 124 matching lines @@
   static final int RESOURCE = 4;
 
   /** Next state. */
   static final List<int> NEXT =
       const <int>[NO_TAG_SEEN,
                   IMPORT, // Only one library tag is allowed.
                   IMPORT,
                   SOURCE,
                   RESOURCE];
 }