Store a file state in the incremental resolved AST generator.

pkg/front_end/test/incremental_resolved_ast_generator_test.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 'dart:async';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_resolution_map.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/file_system/physical_file_system.dart';
@@ skipping 52 matching lines @@
     expect(initialProgram.keys, unorderedEquals([fooUri]));
 
     void _checkMain(CompilationUnit unit, int expectedArgument) {
       var mainStatements = _getFunctionStatements(_getFunction(unit, 'main'));
       expect(mainStatements, hasLength(1));
       _checkPrintLiteralInt(mainStatements[0], expectedArgument);
     }
 
     _checkMain(initialProgram[fooUri].definingCompilationUnit, 1);
     writeFiles({'/foo.dart': 'main() { print(2); }'});
-    // TODO(paulberry): verify that the file isn't actually reread until
-    // invalidate is called.
-    // var deltaProgram1 = await incrementalResolvedAstGenerator.computeDelta();
+    // Verify that the file isn't actually reread until invalidate is called.
+    var deltaProgram1 = await incrementalResolvedAstGenerator.computeDelta();
+    // TODO(paulberry): since there is no delta, computeDelta should return an
+    // empty map.
     // expect(deltaProgram1.newState, isEmpty);
+    expect(deltaProgram1.newState.keys, unorderedEquals([fooUri]));
+    _checkMain(deltaProgram1.newState[fooUri].definingCompilationUnit, 1);
     incrementalResolvedAstGenerator.invalidateAll();
     var deltaProgram2 = await incrementalResolvedAstGenerator.computeDelta();
     expect(deltaProgram2.newState.keys, unorderedEquals([fooUri]));
     _checkMain(deltaProgram2.newState[fooUri].definingCompilationUnit, 2);
   }
 
   test_invalidateAllBeforeInitialProgram() async {
     incrementalResolvedAstGenerator = new IncrementalResolvedAstGenerator(
         Uri.parse('file:///foo.dart'),
         new CompilerOptions()
@@ skipping 74 matching lines @@
     }
     throw fail('No function declaration found with name "$name"');
   }
 
   NodeList<Statement> _getFunctionStatements(FunctionDeclaration function) {
     var body = function.functionExpression.body;
     expect(body, new isInstanceOf<BlockFunctionBody>());
     return (body as BlockFunctionBody).block.statements;
   }
 }