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

lib/compiler/implementation/compiler.dart

Index: lib/compiler/implementation/compiler.dart
diff --git a/lib/compiler/implementation/compiler.dart b/lib/compiler/implementation/compiler.dart
index 02bdebe1cdeedb7544de193cb0afd26726f0a8f7..92ea94a0f818524d53db96e68fdc42fb9d30618e 100644
--- a/lib/compiler/implementation/compiler.dart
+++ b/lib/compiler/implementation/compiler.dart
@@ -349,17 +349,7 @@ class Compiler implements DiagnosticListener {
     coreImplLibrary = scanBuiltinLibrary('coreimpl');
     jsHelperLibrary = scanBuiltinLibrary('_js_helper');
     interceptorsLibrary = scanBuiltinLibrary('_interceptors');
-    coreLibrary = scanBuiltinLibrary('core');
-
-    // Since coreLibrary import the libraries "coreimpl", "js_helper",
-    // and "interceptors", coreLibrary is null when they are being
-    // built. So we add the implicit import of coreLibrary now. This
-    // can be cleaned up when we have proper support for "dart:core"
-    // and don't need to access it through the field "coreLibrary".
-    // TODO(ahe): Clean this up as described above.
-    scanner.importLibrary(coreImplLibrary, coreLibrary, null);
-    scanner.importLibrary(jsHelperLibrary, coreLibrary, null);
-    scanner.importLibrary(interceptorsLibrary, coreLibrary, null);
+
     addForeignFunctions(jsHelperLibrary);
     addForeignFunctions(interceptorsLibrary);
 
@@ -374,6 +364,17 @@ class Compiler implements DiagnosticListener {
     //patchDartLibrary(coreImplLibrary, 'coreimpl');
   }
 
+  void importCoreLibrary(LibraryElement library) {
+    Uri coreUri = new Uri.fromComponents(scheme: 'dart', path: 'core');
+    if (coreLibrary === null) {
+      coreLibrary = scanner.loadLibrary(coreUri, null, coreUri);
+    }
+    scanner.importLibrary(library,
+                          coreLibrary,
+                          null,
+                          library.entryCompilationUnit);
+  }
+
   void patchDartLibrary(LibraryElement library, String dartLibraryPath) {
     if (library.isPatched) return;
     Uri patchUri = resolvePatchUri(dartLibraryPath);