Disallow `null` in spanFromSpannable.

pkg/compiler/lib/src/resolution/class_hierarchy.dart

Index: pkg/compiler/lib/src/resolution/class_hierarchy.dart
diff --git a/pkg/compiler/lib/src/resolution/class_hierarchy.dart b/pkg/compiler/lib/src/resolution/class_hierarchy.dart
index ef03ab58bd5b30874418f11ba593c3df0259ca22..551a47e7332bbed9323fa1546f0b3ac578d0b1dc 100644
--- a/pkg/compiler/lib/src/resolution/class_hierarchy.dart
+++ b/pkg/compiler/lib/src/resolution/class_hierarchy.dart
@@ -482,12 +482,12 @@ class ClassResolverVisitor extends TypeDefinitionVisitor {
       if (interfaceType != null) {
         if (interfaceType.isMalformed) {
           reporter.reportErrorMessage(
-              superclass,
+              link.head,
               MessageKind.CANNOT_IMPLEMENT_MALFORMED,
               {'className': element.name, 'malformedType': interfaceType});
         } else if (interfaceType.isEnumType) {
           reporter.reportErrorMessage(
-              superclass,
+              link.head,
               MessageKind.CANNOT_IMPLEMENT_ENUM,
               {'className': element.name, 'enumType': interfaceType});
         } else if (!interfaceType.isInterfaceType) {