Use failedAt in more places

pkg/compiler/lib/src/compiler.dart

Index: pkg/compiler/lib/src/compiler.dart
diff --git a/pkg/compiler/lib/src/compiler.dart b/pkg/compiler/lib/src/compiler.dart
index 9885f5d8801b3aa7ac9e0b9d671d0292093c1e0a..64405b90222488f263c78bd87332e96caf8dc34e 100644
--- a/pkg/compiler/lib/src/compiler.dart
+++ b/pkg/compiler/lib/src/compiler.dart
@@ -860,13 +860,11 @@ abstract class Compiler {
    * See [LibraryLoader] for terminology on URIs.
    */
   Future<Script> readScript(Uri readableUri, [Spannable node]) {
-    throw new SpannableAssertionFailure(
-        node, 'Compiler.readScript not implemented.');
+    throw failedAt(node, 'Compiler.readScript not implemented.');
   }
 
   Future<Binary> readBinary(Uri readableUri, [Spannable node]) {
-    throw new SpannableAssertionFailure(
-        node, 'Compiler.readBinary not implemented.');
+    throw failedAt(node, 'Compiler.readBinary not implemented.');
   }
 
   Element lookupElementIn(ScopeContainerElement container, String name) {