Add MultiSourceInformationStrategy

tests/compiler/dart2js/sourcemaps/multi_source_info_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:async_helper/async_helper.dart';
+import 'package:compiler/compiler_new.dart';
+import 'package:compiler/src/commandline_options.dart';
+import 'package:expect/expect.dart';
+import '../memory_compiler.dart';
+
+main() {
+  asyncTest(() async {
+    String oldMap = (await compile([])).getOutput('', OutputType.js_map);
+    String newMap = (await compile([Flags.useNewSourceInfo]))
+        .getOutput('', OutputType.js_map);
+    OutputCollector multiCollector1 = await compile([Flags.useMultiSourceInfo]);
+    String multiMap1a = multiCollector1.getOutput('', OutputType.js_map);
+    String multiMap1b = multiCollector1.getOutput('out.js', OutputType.js_map);
+    Expect.equals(oldMap, multiMap1a);
+    Expect.equals(newMap, multiMap1b);
+    OutputCollector multiCollector2 =
+        await compile([Flags.useMultiSourceInfo, Flags.useNewSourceInfo]);
+    String multiMap2a = multiCollector2.getOutput('', OutputType.js_map);
+    String multiMap2b = multiCollector2.getOutput('out.js', OutputType.js_map);
+    Expect.equals(newMap, multiMap2a);
+    Expect.equals(oldMap, multiMap2b);
+  });
+}
+
+Future<OutputCollector> compile(List<String> options) async {
+  OutputCollector collector = new OutputCollector();
+  await runCompiler(
+      entryPoint: Uri.parse('memory:main.dart'),
+      memorySourceFiles: const {'main.dart': 'main() {}'},
+      outputProvider: collector,
+      options: options);
+  return collector;
+}