Make Analyzer, VM and dart2js accept URI strings as part-of library identifier.

pkg/analyzer/lib/src/dart/analysis/library_analyzer.dart

 // Copyright (c) 2017, 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.
 
 import 'package:analyzer/context/declared_variables.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/error/error.dart';
 import 'package:analyzer/error/listener.dart';
@@ skipping 456 matching lines @@
                       partUri,
                       [_library.uriStr, source.uri]);
                 }
               }
             }
           }
         }
       }
     }
 
-    if (hasPartDirective && libraryNameNode == null) {
+    if (hasPartDirective && libraryNameNode == null &&
+        !_context.analysisOptions.enableUriInPartOf) {
       libraryErrorReporter.reportErrorForOffset(
           ResolverErrorCode.MISSING_LIBRARY_DIRECTIVE_WITH_PART, 0, 0);
     }
 
     //
     // Resolve the relevant directives to the library element.
     //
     for (Directive directive in directivesToResolve) {
       directive.element = _libraryElement;
     }
@@ skipping 271 matching lines @@
 }
 
 /**
  * Either the name or the source associated with a part-of directive.
  */
 class _NameOrSource {
   final String name;
   final Source source;
   _NameOrSource(this.name, this.source);
 }