Fix for issue 5223 : Import 'dart:core' implicitly only if it has not been

tests/language/import_core_prefix_test.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.
+// Test explicit import of dart:core in the source code..
+
+#library("ImportCorePrefixTest.dart");
+#import("dart:core", prefix:"mycore");
+
+void main() {
+  var test = new mycore.Map<mycore.int,mycore.String>();
+  mycore.bool boolval = false;
+  mycore.int variable = 10;
+  mycore.num value = 10;
+}