fix errors and warnings in DDC

pkg/dev_compiler/lib/src/compiler/compiler.dart

 // 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.
 
 import 'dart:collection' show HashSet, Queue;
-import 'dart:convert' show BASE64, JSON, UTF8;
+import 'dart:convert' show JSON;
 import 'dart:io' show File;
 
 import 'package:analyzer/analyzer.dart'
     show AnalysisError, CompilationUnit, ErrorSeverity;
 import 'package:analyzer/dart/element/element.dart' show LibraryElement;
 import 'package:analyzer/file_system/file_system.dart' show ResourceProvider;
 import 'package:analyzer/file_system/physical_file_system.dart'
     show PhysicalResourceProvider;
 import 'package:analyzer/src/context/builder.dart' show ContextBuilder;
 import 'package:analyzer/src/context/context.dart' show AnalysisContextImpl;
@@ skipping 579 matching lines @@
   return map;
 }
 
 /// Cleanup the dart_sdk source map.
 ///
 /// Strip out files that should not be included in the sdk sourcemap as they
 /// are implementation details that would just confuse users.
 /// Normalize sdk urls to use "dart:" for more understandable stack traces.
 Map cleanupSdkSourcemap(Map sourceMap) {
   var map = new Map.from(sourceMap);
-  var list = new List.from(map['sources']);
   map['sources'] = map['sources']
       .map((url) => url.contains('/_internal/') ? null : url)
       .toList();
   return map;
 }