Only units with 'part of' and no other directives are parts.

pkg/analyzer/test/src/context/context_test.dart

 // Copyright (c) 2014, 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 test.src.context.context_test;
 
 import 'dart:async';
 
 import 'package:analyzer/src/cancelable_future.dart';
 import 'package:analyzer/src/context/cache.dart';
@@ skipping 546 matching lines @@
   void test_performAnalysisTask_changePartContents_makeItAPart() {
     Source libSource = addSource("/lib.dart", r'''
 library lib;
 part 'part.dart';
 void f(x) {}''');
     Source partSource = addSource("/part.dart", "void g() { f(null); }");
     _analyzeAll_assertFinished();
     expect(context.getResolvedCompilationUnit2(libSource, libSource), isNotNull,
         reason: "library resolved 1");
     expect(
-        context.getResolvedCompilationUnit2(partSource, libSource), isNotNull,
+        context.getResolvedCompilationUnit2(partSource, partSource), isNotNull,
         reason: "part resolved 1");
     // update and analyze
     context.setContents(partSource, r'''
 part of lib;
 void g() { f(null); }''');
     expect(context.getResolvedCompilationUnit2(libSource, libSource), isNull,
         reason: "library changed 2");
-    expect(context.getResolvedCompilationUnit2(partSource, libSource), isNull,
+    expect(context.getResolvedCompilationUnit2(partSource, partSource), isNull,
         reason: "part changed 2");
     _analyzeAll_assertFinished();
     expect(context.getResolvedCompilationUnit2(libSource, libSource), isNotNull,
         reason: "library resolved 2");
     expect(
         context.getResolvedCompilationUnit2(partSource, libSource), isNotNull,
         reason: "part resolved 2");
     expect(context.getErrors(libSource).errors, hasLength(0));
     expect(context.getErrors(partSource).errors, hasLength(0));
   }
@@ skipping 1548 matching lines @@
       : super(name, pathos.toUri(name), UriKind.FILE_URI);
 
   @override
   TimestampedData<String> get contents {
     throw 'Read error';
   }
 
   @override
   bool exists() => true;
 }