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

runtime/vm/object.h

Index: runtime/vm/object.h
===================================================================
--- runtime/vm/object.h	(revision 12667)
+++ runtime/vm/object.h	(working copy)
@@ -2106,6 +2106,7 @@
   intptr_t num_imports() const { return raw_ptr()->num_imports_; }
   RawLibrary* ImportAt(intptr_t index) const;
   RawLibraryPrefix* ImportPrefixAt(intptr_t index) const;
+  bool ImportsCorelib() const;
 
   RawFunction* LookupFunctionInSource(const String& script_url,
                                       intptr_t line_number) const;
@@ -2138,6 +2139,10 @@
     raw_ptr()->debuggable_ = value;
   }
 
+  bool IsCoreLibrary() const {
+    return raw() == CoreLibrary();
+  }
+
   static RawLibrary* LookupLibrary(const String& url);
   static RawLibrary* GetLibrary(intptr_t index);
   static bool IsKeyUsed(intptr_t key);