Enable strong mode in DDC, fix all warnings/errors

lib/src/compiler/type_utilities.dart

 // Copyright (c) 2015, 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 HashMap, HashSet, LinkedHashMap;
 
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 
 import '../js_ast/js_ast.dart' as JS;
@@ skipping 147 matching lines @@
   /// cache/generator variables discharged at the binding site for the
   /// type variable since the type definition depends on the type
   /// parameter.
   final _scopeDependencies =
       new HashMap<TypeParameterElement, List<DartType>>();
 
   /// Emit a list of statements declaring the cache variables and generator
   /// definitions tracked by the table.  If [formals] is present, only
   /// emit the definitions which depend on the formals.
   List<JS.Statement> discharge([List<TypeParameterElement> formals]) {
-    var filter = formals?.expand((p) => _scopeDependencies[p] ?? []);
+    var filter = formals?.expand((p) => _scopeDependencies[p] ?? <DartType>[]);
     var stmts = [
       _cacheNames,
       _definiteCacheNames,
       _generators,
       _definiteGenerators
     ].expand((c) => c.discharge(filter)).toList();
     formals?.forEach(_scopeDependencies.remove);
     return stmts;
   }
 
@@ skipping 44 matching lines @@
     assert(hoistType != null);
     var table = hoistType
         ? (definite ? _definiteGenerators : _generators)
         : (definite ? _definiteCacheNames : _cacheNames);
     if (!table.isNamed(type)) {
       if (recordScopeDependencies(type)) return typeRep;
     }
     return table.nameType(type, typeRep);
   }
 }