Use failedAt in more places (js_emitter)

pkg/compiler/lib/src/js_emitter/code_emitter_task.dart

Index: pkg/compiler/lib/src/js_emitter/code_emitter_task.dart
diff --git a/pkg/compiler/lib/src/js_emitter/code_emitter_task.dart b/pkg/compiler/lib/src/js_emitter/code_emitter_task.dart
index a2b1d97274b3d6607dc38a4844faf0cdf03279d0..6c4cff7bf3a9694068be2c2cad01d6fdc63ed208 100644
--- a/pkg/compiler/lib/src/js_emitter/code_emitter_task.dart
+++ b/pkg/compiler/lib/src/js_emitter/code_emitter_task.dart
@@ -67,14 +67,16 @@ class CodeEmitterTask extends CompilerTask {
   }
 
   NativeEmitter get nativeEmitter {
-    assert(invariant(NO_LOCATION_SPANNABLE, _nativeEmitter != null,
-        message: "NativeEmitter has not been created yet."));
+    assert(
+        _nativeEmitter != null,
+        failedAt(
+            NO_LOCATION_SPANNABLE, "NativeEmitter has not been created yet."));
     return _nativeEmitter;
   }
 
   Emitter get emitter {
-    assert(invariant(NO_LOCATION_SPANNABLE, _emitter != null,
-        message: "Emitter has not been created yet."));
+    assert(_emitter != null,
+        failedAt(NO_LOCATION_SPANNABLE, "Emitter has not been created yet."));
     return _emitter;
   }