fix errors and warnings in DDC

pkg/dev_compiler/lib/src/compiler/code_generator.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;
 import 'dart:math' show min, max;
 
 import 'package:analyzer/analyzer.dart' hide ConstantEvaluator;
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 494 matching lines @@
   /// Because D depends on B, we'll emit B first if needed. However C is not
   /// used by top-level JavaScript code, so we can ignore that dependency.
   void _emitDeclaration(Element e) {
     var item = _loader.emitDeclaration(e, (AstNode node) {
       // TODO(jmesserly): this is not really the right place for this.
       // Ideally we do this per function body.
       //
       // We'll need to be consistent about when we're generating functions, and
       // only run this on the outermost function, and not any closures.
       inferNullableTypes(node);
-      return _visit(node);
+      return _visit(node) as JS.Node;
     });
 
     if (item != null) _moduleItems.add(item);
   }
 
   void _declareBeforeUse(Element e) {
     _loader.declareBeforeUse(e, _emitDeclaration);
   }
 
   void _finishDeclarationsInUnit(CompilationUnit unit) {
@@ skipping 5513 matching lines @@
   if (targetIdentifier.staticElement is! PrefixElement) return false;
   var prefix = targetIdentifier.staticElement as PrefixElement;
 
   // The library the prefix is referring to must come from a deferred import.
   var containingLibrary = resolutionMap
       .elementDeclaredByCompilationUnit(target.root as CompilationUnit)
       .library;
   var imports = containingLibrary.getImportsWithPrefix(prefix);
   return imports.length == 1 && imports[0].isDeferred;
 }