Fix dart2js warnings and add test to ensure it stays clean.

pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart

 // Copyright (c) 2014, 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.
 
 library dart2js.js_emitter.full_emitter;
 
 import 'dart:collection' show HashMap;
 import 'dart:convert';
 
 import 'package:js_runtime/shared/embedded_names.dart' as embeddedNames;
@@ skipping 15 matching lines @@
         FieldElement,
         FunctionElement,
         FunctionSignature,
         LibraryElement,
         MethodElement,
         TypedefElement,
         VariableElement;
 import '../../elements/entities.dart';
 import '../../hash/sha1.dart' show Hasher;
 import '../../io/code_output.dart';
-import '../../io/line_column_provider.dart'
-    show LineColumnCollector, LineColumnProvider;
+import '../../io/line_column_provider.dart' show LineColumnCollector;
 import '../../io/source_map_builder.dart' show SourceMapBuilder;
 import '../../js/js.dart' as jsAst;
 import '../../js/js.dart' show js;
 import '../../js_backend/backend_helpers.dart' show BackendHelpers;
 import '../../js_backend/js_backend.dart'
     show
         ConstantEmitter,
         JavaScriptBackend,
         JavaScriptConstantCompiler,
         Namer,
@@ skipping 1881 matching lines @@
     // data.
     mapping["_comment"] = "This mapping shows which compiled `.js` files are "
         "needed for a given deferred library import.";
     mapping.addAll(compiler.deferredLoadTask.computeDeferredMap());
     compiler.outputProvider(
         compiler.options.deferredMapUri.path, '', OutputType.info)
       ..add(const JsonEncoder.withIndent("  ").convert(mapping))
       ..close();
   }
 }