fix #27320, better DDC temp generation

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 2634 matching lines @@
     return annotate(_toArrowFunction(fn), node);
   }
 
   JS.Fun _makeGenericFunction(JS.Fun fn) {
     if (fn.typeParams == null || fn.typeParams.isEmpty) return fn;
 
     return new JS.Fun(
         fn.typeParams,
         new JS.Block([
           // Convert the function to an => function, to ensure `this` binding.
-          new JS.Return(_toArrowFunction(fn))
+          _toArrowFunction(fn).toReturn()
         ]));
   }
 
   JS.ArrowFun _toArrowFunction(JS.Fun f) {
     JS.Node body = f.body;
 
     // Simplify `=> { return e; }` to `=> e`
     if (body is JS.Block) {
       JS.Block block = body;
       if (block.statements.length == 1) {
@@ skipping 749 matching lines @@
     var body = js.call('# == null ? null : #',
         [_visit(left), _emitSet(_stripNullAwareOp(node, left), right)]);
     return new JS.MetaLet(vars, [body]);
   }
 
   @override
   JS.Block visitExpressionFunctionBody(ExpressionFunctionBody node) {
     var savedFunction = _currentFunction;
     _currentFunction = node;
     var initArgs = _emitArgumentInitializers(node.parent);
-    var ret = annotate(new JS.Return(_visit(node.expression)), node.expression);
+    var ret = annotate(
+        _visit<JS.Expression>(node.expression).toReturn(), node.expression);
     _currentFunction = savedFunction;
     var _statements = initArgs != null ? [initArgs, ret] : [ret];
     var block = annotate(new JS.Block(_statements), node);
     return block;
   }
 
   @override
   JS.Block visitEmptyFunctionBody(EmptyFunctionBody node) => new JS.Block([]);
 
   @override
@@ skipping 2543 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;
 }