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

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

Index: pkg/analyzer/lib/src/dart/analysis/library_analyzer.dart
diff --git a/pkg/analyzer/lib/src/dart/analysis/library_analyzer.dart b/pkg/analyzer/lib/src/dart/analysis/library_analyzer.dart
index 7ac3b30239b911b90d10e7288286085dd018cc04..71f1f2093d0322979dfa9a94e84f8052e05aeab0 100644
--- a/pkg/analyzer/lib/src/dart/analysis/library_analyzer.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/library_analyzer.dart
@@ -474,7 +474,8 @@ class LibraryAnalyzer {
       }
     }
 
-    if (hasPartDirective && libraryNameNode == null) {
+    if (hasPartDirective && libraryNameNode == null &&
+        !_context.analysisOptions.enableUriInPartOf) {
       libraryErrorReporter.reportErrorForOffset(
           ResolverErrorCode.MISSING_LIBRARY_DIRECTIVE_WITH_PART, 0, 0);
     }