Fix various semantic nits that Fasta will detect.

pkg/front_end/test/src/incremental/mock_sdk.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 'package:front_end/memory_file_system.dart';
 
 /// Create SDK libraries which are used by Fasta to perform kernel generation.
 /// Return the mapping from the simple names of these library to the URIs
 /// in the given [fileSystem].  The root of the SDK is `file:///sdk`.
 Map<String, Uri> createSdkFiles(MemoryFileSystem fileSystem) {
   Map<String, Uri> dartLibraries = {};
 
   void addSdkLibrary(String name, String contents) {
     String path = '$name/$name.dart';
     Uri uri = Uri.parse('file:///sdk/lib/$path');
     fileSystem.entityForUri(uri).writeAsStringSync(contents);
     dartLibraries[name] = uri;
   }
 
   addSdkLibrary(
       'core',
       r'''
 library dart.core;
 import 'dart:_internal';
 import 'dart:async';
 
 class Object {
-  const Object() {}
+  const Object();

[ahe, 2017/05/23 14:44:27]

A const constructor can't have a body.

   bool operator ==(other) => identical(this, other);
   String toString() => 'a string';
   int get hashCode => 0;
   Type get runtimeType => null;
   dynamic noSuchMethod(Invocation invocation) => null;
 }
 
 class Null {}
 
 class Symbol {
@@ skipping 229 matching lines @@
 class Symbol {}
 class ExternalName {
   final String name;
   const ExternalName(this.name);
 }
 ''');
   addSdkLibrary('_vmservice', 'library dart._vmservice;');
 
   return dartLibraries;
 }