Use failedAt in more places

pkg/compiler/lib/src/kernel/kernel.dart

Index: pkg/compiler/lib/src/kernel/kernel.dart
diff --git a/pkg/compiler/lib/src/kernel/kernel.dart b/pkg/compiler/lib/src/kernel/kernel.dart
index 4e45653c41125ec58d0c7377daf73b4f5756562b..a7fd2ec95a1d935dc9d4e8d1f075fc5e41864376 100644
--- a/pkg/compiler/lib/src/kernel/kernel.dart
+++ b/pkg/compiler/lib/src/kernel/kernel.dart
@@ -707,12 +707,15 @@ class Kernel {
         compiler.libraryLoader.lookupLibrary(Uris.dart_core);
     ClassElement cls = library.implementation.localLookup(className);
     cls.ensureResolved(compiler.resolution);
-    assert(invariant(CURRENT_ELEMENT_SPANNABLE, cls != null,
-        message: 'dart:core class $className not found.'));
+    assert(
+        cls != null,
+        failedAt(CURRENT_ELEMENT_SPANNABLE,
+            'dart:core class $className not found.'));
     ConstructorElement constructor = cls.lookupConstructor(constructorName);
-    assert(invariant(CURRENT_ELEMENT_SPANNABLE, constructor != null,
-        message: "Constructor '$constructorName' not found "
-            "in class '$className'."));
+    assert(
+        constructor != null,
+        failedAt(CURRENT_ELEMENT_SPANNABLE,
+            "Constructor '$constructorName' not found in class '$className'."));
     compiler.resolution.ensureResolved(constructor);
     return functionToIr(constructor);
   }
@@ -721,8 +724,10 @@ class Kernel {
     LibraryElement library =
         compiler.libraryLoader.lookupLibrary(Uris.dart_core);
     Element function = library.implementation.localLookup(name);
-    assert(invariant(CURRENT_ELEMENT_SPANNABLE, function != null,
-        message: "dart:core method '$name' not found."));
+    assert(
+        function != null,
+        failedAt(
+            CURRENT_ELEMENT_SPANNABLE, "dart:core method '$name' not found."));
     compiler.resolution.ensureResolved(function);
     return functionToIr(function);
   }