Don't crash when reporting an error

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 e6d214c4ddf9faabf7493a9c3303b6eab1439a1f..9ed9c8c06469ae66c640562515b544587da42176 100644
--- a/pkg/compiler/lib/src/compiler.dart
+++ b/pkg/compiler/lib/src/compiler.dart
@@ -1956,6 +1956,10 @@ abstract class Compiler implements DiagnosticListener {
     if (element == null) {
       element = currentElement;
     }
+    if (element == null) {
+      return null;
+    }
+
     if (element.sourcePosition != null) {
       return element.sourcePosition;
     }