Use failedAt in more places

pkg/compiler/lib/src/elements/modelx.dart

 // Copyright (c) 2013, 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 elements.modelx;
 
 import '../common.dart';
 import '../common/names.dart' show Identifiers;
 import '../common/resolution.dart' show Resolution, ParsingContext;
 import '../compiler.dart' show Compiler;
@@ skipping 1552 matching lines @@
         SendSet sendSet = initializedIdentifier.asSendSet();
         identifier = sendSet.selector.asIdentifier();
         if (identical(name, identifier.source)) {
           definitionCache = initializedIdentifier;
           initializerCache = sendSet.arguments.first;
         }
       } else if (identical(name, identifier.source)) {
         definitionCache = initializedIdentifier;
       }
     }
-    invariant(definitions, definitionCache != null,
-        message: "Could not find '$name'.");
+    if (definitionCache == null) {
+      failedAt(definitionCache, "Could not find '$name'.");
+    }
     definitionsCache = definitions;
   }
 
   ResolutionDartType computeType(Resolution resolution) {
     if (variables.type != null) return variables.type;
     // Call [parseNode] to ensure that [definitionsCache] and [initializerCache]
     // are set as a consequence of calling [computeType].
     parseNode(resolution.parsingContext);
     return variables.computeType(this, resolution);
   }
@@ skipping 1927 matching lines @@
       body = node.asFunctionExpression().body;
     }
     return new ParsedResolvedAst(
         declaration,
         node,
         body,
         definingElement.treeElements,
         definingElement.compilationUnit.script.resourceUri);
   }
 }