Use failedAt in more places (misc)

pkg/compiler/lib/src/diagnostics/invariant.dart

Index: pkg/compiler/lib/src/diagnostics/invariant.dart
diff --git a/pkg/compiler/lib/src/diagnostics/invariant.dart b/pkg/compiler/lib/src/diagnostics/invariant.dart
index 34a9f3e9ffaefbc15e246ea8f466cc643906b487..eb31d04b35a448af833c5de144fc06ccb9b50730 100644
--- a/pkg/compiler/lib/src/diagnostics/invariant.dart
+++ b/pkg/compiler/lib/src/diagnostics/invariant.dart
@@ -22,8 +22,8 @@ bool DEBUG_MODE = false;
 /// Assert that [DEBUG_MODE] is `true` and provide [message] as part of the
 /// error message.
 assertDebugMode(String message) {
-  assert(invariant(NO_LOCATION_SPANNABLE, DEBUG_MODE,
-      message: 'Debug mode is not enabled: $message'));
+  assert(DEBUG_MODE,
+      failedAt(NO_LOCATION_SPANNABLE, 'Debug mode is not enabled: $message'));
 }
 
 /**