Repeats and fixes the changes landed & reverted as CL 1789553003.

tests/compiler/dart2js/library_env_test.dart

-// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// Copyright (c) 2016, 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.
 
 /// Check that 'dart:' libraries have their corresponding dart.library.X
 /// environment variable set.
 
-import "dart:io";
-
-import "dart:async";
-
 import "memory_source_file_helper.dart";
 
 import "package:async_helper/async_helper.dart";
 
 import 'package:expect/expect.dart' show
     Expect;
 
-import 'package:compiler/src/elements/elements.dart' show
-    LibraryElement;
-
 import 'package:compiler/src/null_compiler_output.dart' show
     NullCompilerOutput;
 
+import 'package:compiler/src/options.dart' show
+    CompilerOptions;
+
 import 'package:compiler/compiler_new.dart' show
     CompilerInput,
-    CompilerDiagnostics,
-    CompilerOptions;
-
-import 'package:sdk_library_metadata/libraries.dart' show
-    LibraryInfo;
+    CompilerDiagnostics;
 
 const clientPlatform = r'''
 [dart-spec]
 spec: 3rd edition.
 
 [features]
 # No extra features
 
 [libraries]
 mock.client: mock1.dart
@@ skipping 95 matching lines @@
   Expect.equals("false", compiler.fromEnvironment("dart.library.collection"));
   Expect.equals("foo", compiler.fromEnvironment("dart.library.mock.client"));
 }
 
 main() {
   asyncStart();
   runTest().then((_) {
     asyncEnd();
   });
 }