A couple of tweaks in ValidatingInstrumentation.

pkg/front_end/lib/src/fasta/testing/validating_instrumentation.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.md file.
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
 import 'package:front_end/src/base/instrumentation.dart';
 import 'package:front_end/src/fasta/messages.dart';
 import 'package:front_end/src/fasta/scanner.dart';
@@ skipping 34 matching lines @@
 
   /// Indicates whether any expectation mismatches were found.
   ///
   /// Should be called after [finish].
   bool get hasProblems => _problems.isNotEmpty;
 
   /// Gets a description of all expectation mismatches that were found, in a
   /// form suitable for printing to the console.
   ///
   /// Should be called after [finish].
-  get problemsAsString => _problems.join('\n');
+  String get problemsAsString => _problems.join('\n');
 
   /// Checks whether the property/value pairs passed to [record] match the
   /// expectations loaded by [loadExpectations].
   void finish() {
     _unsatisfiedExpectations.forEach((uri, expectationsForUri) {
       expectationsForUri.forEach((offset, expectationsAtOffset) {
         for (var expectation in expectationsAtOffset) {
           _problem(
               uri,
               offset,
               'expected ${expectation.property}=${expectation.value}, '
               'got nothing',
               new _Fix(
                   expectation.commentOffset, expectation.commentLength, ''));
         }
       });
     });
   }
 
   /// Updates the source file at [uri] based on the actual property/value
   /// pairs that were observed.
   Future<Null> fixSource(Uri uri) async {
     var fixes = _fixes[uri];
     if (fixes == null) return;
-    var bytes =
-        (await readBytesFromFile(uri, ensureZeroTermination: false)).toList();
+    File file = new File.fromUri(uri);
+    var bytes = await file.readAsBytes();
     // Apply the fixes in reverse order so that offsets don't need to be
     // adjusted after each fix.
     fixes.sort((a, b) => b.offset.compareTo(a.offset));
     for (var fix in fixes) {
       bytes.replaceRange(
           fix.offset, fix.offset + fix.length, UTF8.encode(fix.replacement));
     }
-    await new File.fromUri(uri).writeAsBytes(bytes);
+    await file.writeAsBytes(bytes);
   }
 
   /// Loads expectations from the source file located at [uri].
   ///
   /// Should be called before [finish].
   Future<Null> loadExpectations(Uri uri) async {
     var bytes = await readBytesFromFile(uri);
     var expectations = _unsatisfiedExpectations.putIfAbsent(uri, () => {});
     var testedFeaturesState = _testedFeaturesState.putIfAbsent(uri, () => {});
     ScannerResult result = scan(bytes, includeComments: true);
@@ skipping 120 matching lines @@
       this.property, this.value, this.commentOffset, this.commentLength);
 }
 
 class _Fix {
   final int offset;
   final int length;
   final String replacement;
 
   _Fix(this.offset, this.length, this.replacement);
 }