Issue a more meaningful error message when we see duplicate definitions (ambiguous reference)

vm/parser.cc

 // 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.
 
 #include "vm/parser.h"
 
 #include "vm/bigint_operations.h"
 #include "vm/class_finalizer.h"
 #include "vm/compiler.h"
 #include "vm/compiler_stats.h"
@@ skipping 7968 matching lines @@
     String& first_lib_url = String::Handle();
     Library& lib = Library::Handle();
     intptr_t num_imports = library_.num_imports();
     Object& resolved_obj = Object::Handle();
     for (int i = 0; i < num_imports; i++) {
       lib ^= library_.ImportAt(i);
       resolved_obj = LookupNameInLibrary(lib, name);
       if (!resolved_obj.IsNull()) {
         if (!first_lib_url.IsNull()) {
           // Found duplicate definition.
-          ErrorMsg(ident_pos,
-                   "ambiguous reference: "
-                   "'%s' is defined in library '%s' and also in '%s'",
-                   name.ToCString(),
-                   first_lib_url.ToCString(),
-                   String::Handle(lib.url()).ToCString());
+          if (first_lib_url.raw() == lib.url()) {
+            ErrorMsg(ident_pos,
+                     "ambiguous reference: "
+                     "'%s' as library '%s' is imported multiple times",
+                     name.ToCString(),
+                     first_lib_url.ToCString());
+          } else {
+            ErrorMsg(ident_pos,
+                     "ambiguous reference: "
+                     "'%s' is defined in library '%s' and also in '%s'",
+                     name.ToCString(),
+                     first_lib_url.ToCString(),
+                     String::Handle(lib.url()).ToCString());
+          }
         } else {
           first_lib_url = lib.url();
           obj = resolved_obj.raw();
         }
       }
     }
   }
   return obj.raw();
 }
 
@@ skipping 1505 matching lines @@
 void Parser::SkipQualIdent() {
   ASSERT(IsIdentifier());
   ConsumeToken();
   if (CurrentToken() == Token::kPERIOD) {
     ConsumeToken();  // Consume the kPERIOD token.
     ExpectIdentifier("identifier expected after '.'");
   }
 }
 
 }  // namespace dart