Enable strong mode in DDC, fix all warnings/errors

bin/dartdevc.dart

 #!/usr/bin/env 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.
 
 /// Command line entry point for Dart Development Compiler (dartdevc).
 ///
 /// Supported commands are
 ///   * compile: builds a collection of dart libraries into a single JS module
 ///
@@ skipping 57 matching lines @@
 
 /// Runs the compiler worker loop.
 class _CompilerWorker extends AsyncWorkerLoop {
   /// The original args supplied to the executable.
   final List<String> _startupArgs;
 
   _CompilerWorker(this._startupArgs) : super();
 
   /// Performs each individual work request.
   Future<WorkResponse> performRequest(WorkRequest request) async {
-    var args = new List.from(_startupArgs)..addAll(request.arguments);
+    var args = _startupArgs.toList()..addAll(request.arguments);
 
     var output = new StringBuffer();
     var exitCode = await _runCommand(args, messageHandler: output.writeln);
     AnalysisEngine.instance.clearCaches();
     return new WorkResponse()
       ..exitCode = exitCode
       ..output = output.toString();
   }
 }
 
@@ skipping 47 matching lines @@
 /// Bazel actions that support workers must provide all their per-WorkRequest
 /// arguments in a file like this instead of as normal args.
 List<String> _preprocessArgs(List<String> args) {
   args = new List.from(args);
   if (args.isNotEmpty && args.last.startsWith('@')) {
     var fileArg = args.removeLast();
     args.addAll(new File(fileArg.substring(1)).readAsLinesSync());
   }
   return args;
 }