dart2js: replace 'invariant' with 'failedAt' to defer interpolation

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 7ad6f3580a0070cd7febe9c6e03aac04c6817b53..34a9f3e9ffaefbc15e246ea8f466cc643906b487 100644
--- a/pkg/compiler/lib/src/diagnostics/invariant.dart
+++ b/pkg/compiler/lib/src/diagnostics/invariant.dart
@@ -44,7 +44,7 @@ bool invariant(Spannable spannable, var condition, {var message: null}) {
   // information on assertion errors.
   if (spannable == null) {
     throw new SpannableAssertionFailure(CURRENT_ELEMENT_SPANNABLE,
-        "Spannable was null for invariant. Use CURRENT_ELEMENT_SPANNABLE.");
+        'Spannable was null for invariant. Use CURRENT_ELEMENT_SPANNABLE.');
   }
   if (condition is Function) {
     condition = condition();
@@ -62,4 +62,28 @@ bool invariant(Spannable spannable, var condition, {var message: null}) {
   return true;
 }
 
+/// Throws a [SpannableAssertionFailure].
+///
+/// Use this method to provide better information for assertion by calling
+/// [failedAt] as the second argument to an `assert` statement:
+///
+///     assert(condition, failedAt(position, message));
+///
+/// [spannable] must be non-null and will be used to provide positional
+/// information in the generated error message.
+bool failedAt(Spannable spannable, [var message]) {
+  // TODO(johnniwinther): Use [spannable] and [message] to provide better
+  // information on assertion errors.
+  if (spannable == null) {
+    throw new SpannableAssertionFailure(
+        CURRENT_ELEMENT_SPANNABLE,
+        'Spannable was null for failedInvariant. '
+        'Use CURRENT_ELEMENT_SPANNABLE.');
+  }
+  if (message is Function) {
+    message = message();
+  }
+  throw new SpannableAssertionFailure(spannable, message);
+}
+
 typedef void InternalErrorFunction(Spannable location, String message);