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

pkg/analyzer/test/generated/parser_test.dart

Index: pkg/analyzer/test/generated/parser_test.dart
diff --git a/pkg/analyzer/test/generated/parser_test.dart b/pkg/analyzer/test/generated/parser_test.dart
index 0f2780cb0e524e9ebe036071531434d8a50b9aab..0390a1f4c37610b79e57186451c409a76bc2dbb6 100644
--- a/pkg/analyzer/test/generated/parser_test.dart
+++ b/pkg/analyzer/test/generated/parser_test.dart
@@ -3460,7 +3460,8 @@ class Foo {
 
   void test_nonIdentifierLibraryName_partOf() {
     CompilationUnit unit = parseCompilationUnit(
-        "part of 'lib';", [ParserErrorCode.NON_IDENTIFIER_LIBRARY_NAME]);
+        "part of 3;", [ParserErrorCode.MISSING_NAME_IN_PART_OF_DIRECTIVE,
+                       ParserErrorCode.UNEXPECTED_TOKEN]);
     expect(unit, isNotNull);
   }