Make dart2js understand relative part-of in platform libraries.

pkg/compiler/lib/src/elements/modelx.dart

 // Copyright (c) 2013, 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.
 
 library elements.modelx;
 
 import '../common.dart';
 import '../common/names.dart' show Identifiers;
 import '../common/resolution.dart' show Resolution, ParsingContext;
 import '../compiler.dart' show Compiler;
@@ skipping 764 matching lines @@
     }
     partTag = tag;
     LibraryName libraryTag = library.libraryTag;
 
     Expression libraryReference = tag.name;
     if (libraryReference is LiteralString) {
       // Name is a URI. Resolve and compare to library's URI.
       String content = libraryReference.dartString.slowToString();
       Uri uri = this.script.readableUri.resolve(content);
       Uri expectedUri = library.canonicalUri;
+      // Also allow `string.dart` to refer to `dart:core` as `core.dart`.
+      if (library.isPlatformLibrary && !uri.isScheme("dart")) {
+        expectedUri = library.entryCompilationUnit.script.readableUri;
+      }
       if (uri != expectedUri) {
         // Consider finding a relative URI reference for the error message.
         reporter.reportWarningMessage(tag.name,
             MessageKind.LIBRARY_URI_MISMATCH, {'libraryUri': expectedUri});
       }
       return;
     }
     String actualName = tag.name.toString();
     if (libraryTag != null) {
       String expectedName = libraryTag.name.toString();
@@ skipping 2717 matching lines @@
       body = node.asFunctionExpression().body;
     }
     return new ParsedResolvedAst(
         declaration,
         node,
         body,
         definingElement.treeElements,
         definingElement.compilationUnit.script.resourceUri);
   }
 }