Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/source_mapping_test.dart

 // Copyright (c) 2012, 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:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/io/code_output.dart';
 import 'package:compiler/src/io/source_file.dart';
 import 'package:compiler/src/io/source_information.dart';
 import 'package:compiler/src/js_backend/js_backend.dart';
 import 'package:compiler/src/js_emitter/full_emitter/emitter.dart' as full
     show Emitter;
 
 import 'mock_compiler.dart';
 
 Future<CodeBuffer> compileAll(SourceFile sourceFile) {
   MockCompiler compiler = new MockCompiler.internal();
   Uri uri = new Uri(path: sourceFile.filename);
   compiler.sourceFiles[uri.toString()] = sourceFile;
   JavaScriptBackend backend = compiler.backend;
   return compiler.run(uri).then((_) {
     // TODO(floitsch): the outputBuffers are only accessible in the full
     // emitter.
     full.Emitter fullEmitter = backend.emitter.emitter;
+    // CodeOutput isn't assignable to CodeBuffer.
+    // ignore: RETURN_OF_INVALID_TYPE
     return fullEmitter.outputBuffers[compiler.deferredLoadTask.mainOutputUnit];
   });
 }
 
 void testSourceMapLocations(String codeWithMarkers) {
   List<int> expectedLocations = new List<int>();
   for (int i = 0; i < codeWithMarkers.length; ++i) {
     if (codeWithMarkers[i] == '@') {
       expectedLocations.add(i - expectedLocations.length);
     }
@@ skipping 78 matching lines @@
   testSourceMapLocations(FUNCTIONS_TEST);
   testSourceMapLocations(RETURN_TEST);
   testSourceMapLocations(NOT_TEST);
   testSourceMapLocations(UNARY_TEST);
   testSourceMapLocations(BINARY_TEST);
   testSourceMapLocations(SEND_TEST);
   testSourceMapLocations(SEND_SET_TEST);
   testSourceMapLocations(LOOP_TEST);
   testSourceMapLocations(INTERCEPTOR_TEST);
 }