Use failedAt in more places

pkg/compiler/lib/src/js_emitter/program_builder/field_visitor.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.
 
 part of dart2js.js_emitter.program_builder;
 
 /**
  * [member] is a field (instance, static, or top level).
  *
  * [name] is the field name that the [Namer] has picked for this field's
@@ skipping 61 matching lines @@
       isNativeClass = _nativeData.isNativeClass(cls);
 
       // If the class is never instantiated we still need to set it up for
       // inheritance purposes, but we can simplify its JavaScript constructor.
       isInstantiated =
           _codegenWorldBuilder.directlyInstantiatedClasses.contains(cls);
     } else if (library != null) {
       isLibrary = true;
       assert(visitStatics, failedAt(library));
     } else {
-      throw new SpannableAssertionFailure(NO_LOCATION_SPANNABLE,
+      failedAt(NO_LOCATION_SPANNABLE,
           'Expected a ClassElement or a LibraryElement.');
     }
 
     void visitField(FieldEntity field, {ClassEntity holder}) {
       assert(!(field is FieldElement && !field.isDeclaration), failedAt(field));
 
       bool isMixinNativeField =
           isNativeClass && _elementEnvironment.isMixinApplication(holder);
 
       // See if we can dynamically create getters and setters.
@@ skipping 82 matching lines @@
         field is ClosureFieldElement;
   }
 
   bool canAvoidGeneratedCheckedSetter(FieldElement member) {
     // We never generate accessors for top-level/static fields.
     if (!member.isInstanceMember) return true;
     ResolutionDartType type = member.type;
     return type.treatAsDynamic || type.isObject;
   }
 }