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

pkg/analyzer/lib/src/summary/summarize_ast.dart

 // Copyright (c) 2016, 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 serialization.summarize_ast;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 import 'package:analyzer/dart/element/type.dart' show DartType;
@@ skipping 1354 matching lines @@
   @override
   void visitPartDirective(PartDirective node) {
     parts.add(new UnlinkedPartBuilder(
         uriOffset: node.uri.offset,
         uriEnd: node.uri.end,
         annotations: serializeAnnotations(node.metadata)));
   }
 
   @override
   void visitPartOfDirective(PartOfDirective node) {
-    isCoreLibrary = node.libraryName.name == 'dart.core';
+    isCoreLibrary = node.libraryName?.name == 'dart.core';
     isPartOf = true;
   }
 
   @override
   UnlinkedParamBuilder visitSimpleFormalParameter(SimpleFormalParameter node) {
     UnlinkedParamBuilder b = serializeParameter(node);
     b.type = serializeTypeName(node.type);
     return b;
   }
 
@@ skipping 35 matching lines @@
 /**
  * A [_TypeParameterScope] is a [_Scope] which defines [_ScopedTypeParameter]s.
  */
 class _TypeParameterScope extends _Scope {
   /**
    * Get the number of [_ScopedTypeParameter]s defined in this
    * [_TypeParameterScope].
    */
   int get length => _definedNames.length;
 }