Support imports for dart: libs.

lib/compiler/implementation/dart_backend/backend.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 BailoutException {
   final String reason;
 
   const BailoutException(this.reason);
 }
 
@@ skipping 46 matching lines @@
     }
 
     /**
      * Tells whether we should output given element. Corelib classes like
      * Object should not be in the resulting code.
      */
     final LIBS_TO_IGNORE = [
       compiler.jsHelperLibrary,
       compiler.interceptorsLibrary,
     ];
-    compiler.libraries.forEach((uri, lib) {
-      if (uri.startsWith('dart:')) LIBS_TO_IGNORE.add(lib);
-    });
     bool shouldOutput(Element element) =>
       element.kind !== ElementKind.VOID &&
-      LIBS_TO_IGNORE.indexOf(element.getLibrary()) == -1;
+      LIBS_TO_IGNORE.indexOf(element.getLibrary()) == -1 &&
+      !isDartCoreLib(compiler, element.getLibrary());
 
     try {
       Emitter emitter = new Emitter(compiler);
       resolvedElements.forEach((element, treeElements) {
         if (!shouldOutput(element)) return;
         if (element.isMember()) {
           var enclosingClass = element.enclosingElement;
           assert(enclosingClass.isClass());
           assert(enclosingClass.isTopLevel());
           addMemberToClass(element, enclosingClass);
@@ skipping 13 matching lines @@
       compiler.assembledCode = '''
 main() {
   final bailout_reason = "${e.reason}";
 }
 ''';
     }
   }
 
   log(String message) => compiler.log('[DartBackend] $message');
 }
+
+/**
+ * Checks if [:libraryElement:] is a core lib, that is a library
+ * provided by the implementation like dart:core, dart:coreimpl, etc.
+ */
+bool isDartCoreLib(Compiler compiler, LibraryElement libraryElement) {
+  final libraries = compiler.libraries;
+  for (final uri in libraries.getKeys()) {
+    if (libraryElement === libraries[uri]) {
+      if (uri.startsWith('dart:')) return true;
+    }
+  }
+  return false;
+}