Use failedAt in more places

pkg/compiler/lib/src/apiimpl.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 leg_apiimpl;
 
 import 'dart:async';
 
 import 'package:package_config/packages.dart';
 import 'package:package_config/packages_file.dart' as pkgs;
@@ skipping 359 matching lines @@
   final Map<String, String> definitions;
 
   // TODO(sigmund): break the circularity here: Compiler needs an environment to
   // initialize the library loader, but the environment here needs to know about
   // how the sdk is set up and about whether the backend supports mirrors.
   CompilerImpl compiler;
 
   _Environment(this.definitions);
 
   String valueOf(String name) {
-    assert(invariant(
-        NO_LOCATION_SPANNABLE, compiler.resolvedUriTranslator != null,
-        message: "setupSdk() has not been run"));
+    assert(compiler.resolvedUriTranslator != null,
+        failedAt(NO_LOCATION_SPANNABLE, "setupSdk() has not been run"));
 
     var result = definitions[name];
     if (result != null || definitions.containsKey(name)) return result;
     if (!name.startsWith(_dartLibraryEnvironmentPrefix)) return null;
 
     String libraryName = name.substring(_dartLibraryEnvironmentPrefix.length);
 
     // Private libraries are not exposed to the users.
     if (libraryName.startsWith("_")) return null;
 
@@ skipping 24 matching lines @@
   }
 }
 
 /// For every 'dart:' library, a corresponding environment variable is set
 /// to "true". The environment variable's name is the concatenation of
 /// this prefix and the name (without the 'dart:'.
 ///
 /// For example 'dart:html' has the environment variable 'dart.library.html' set
 /// to "true".
 const String _dartLibraryEnvironmentPrefix = 'dart.library.';