restore FutureOr cast failure ignore

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 1106 matching lines @@
             '  if (o === true || o === false || o == null) return o;'
             '  return #.as(o, #, true);'
             '}',
             [className, _runtimeModule, className]));
         return null;
       }
     }
     if (classElem.library.isDartAsync) {
       if (classElem == types.futureOrType.element) {
         var typeParamT = classElem.typeParameters[0].type;
-        var tOrFutureOfT = js.call('#.is(o) || #.is(o)', [
-          _emitType(typeParamT),
-          _emitType(types.futureType.instantiate([typeParamT]))
-        ]);
+        var typeT = _emitType(typeParamT);
+        var futureOrT = _emitType(types.futureType.instantiate([typeParamT]));
         body.add(js.statement('''
             #.is = function is_FutureOr(o) {
-              return #;
+              return #.is(o) || #.is(o);
             }
-            ''', [className, tOrFutureOfT]));
+            ''', [className, typeT, futureOrT]));
+        // TODO(jmesserly): remove the fallback to `dart.as`. It's only for the
+        // _ignoreTypeFailure logic.
         body.add(js.statement('''
             #.as = function as_FutureOr(o) {
-              if (o == null || #) return o;
-              return #.castError(o, this, false);
+              if (o == null || #.is(o) || #.is(o)) return o;
+              return #.as(o, this, false);
             }
-            ''', [className, tOrFutureOfT, _runtimeModule]));
+            ''', [className, typeT, futureOrT, _runtimeModule]));
         body.add(js.statement('''
             #._check = function check_FutureOr(o) {
-              if (o == null || #) return o;
-              return #.castError(o, this, true);
+              if (o == null || #.is(o) || #.is(o)) return o;
+              return #.as(o, this, true);
             }
-            ''', [className, tOrFutureOfT, _runtimeModule]));
+            ''', [className, typeT, futureOrT, _runtimeModule]));
         return null;
       }
     }
 
     body.add(_callHelperStatement('addTypeTests(#);', [className]));
 
     if (classElem.typeParameters.isEmpty) return null;
 
     // For generics, testing against the default instantiation is common,
     // so optimize that.
@@ skipping 4907 matching lines @@
   // 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;
 }
 
 bool _annotatedNullCheck(Element e) =>
     e != null && findAnnotation(e, isNullCheckAnnotation) != null;