Use failedAt in more places (js_emitter)

pkg/compiler/lib/src/js_emitter/code_emitter_task.dart

 // Copyright (c) 2012, 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.
 
 library dart2js.js_emitter.code_emitter_task;
 
 import 'package:js_runtime/shared/embedded_names.dart' show JsBuiltin;
 
 import '../common.dart';
 import '../common/tasks.dart' show CompilerTask;
@@ skipping 49 matching lines @@
     } else if (useStartupEmitter) {
       _emitterFactory = new startup_js_emitter.EmitterFactory(
           generateSourceMap: generateSourceMap);
     } else {
       _emitterFactory = new full_js_emitter.EmitterFactory(
           generateSourceMap: generateSourceMap);
     }
   }
 
   NativeEmitter get nativeEmitter {
-    assert(invariant(NO_LOCATION_SPANNABLE, _nativeEmitter != null,
-        message: "NativeEmitter has not been created yet."));
+    assert(
+        _nativeEmitter != null,
+        failedAt(
+            NO_LOCATION_SPANNABLE, "NativeEmitter has not been created yet."));
     return _nativeEmitter;
   }
 
   Emitter get emitter {
-    assert(invariant(NO_LOCATION_SPANNABLE, _emitter != null,
-        message: "Emitter has not been created yet."));
+    assert(_emitter != null,
+        failedAt(NO_LOCATION_SPANNABLE, "Emitter has not been created yet."));
     return _emitter;
   }
 
   /// Returns the [Sorter] use for ordering elements in the generated
   /// JavaScript.
   // TODO(johnniwinther): Switch this based on the used entity model.
   Sorter get sorter => const ElementSorter();
 
   String get name => 'Code emitter';
 
@@ skipping 191 matching lines @@
 
   /// Returns the JS code for accessing the given [constant].
   jsAst.Expression constantReference(ConstantValue constant);
 
   /// Returns the JS template for the given [builtin].
   jsAst.Template templateForBuiltin(JsBuiltin builtin);
 
   /// Returns the size of the code generated for a given output [unit].
   int generatedSize(OutputUnit unit);
 }