Guard against NPE

pkg/analyzer/lib/src/task/strong_mode.dart

Index: pkg/analyzer/lib/src/task/strong_mode.dart
diff --git a/pkg/analyzer/lib/src/task/strong_mode.dart b/pkg/analyzer/lib/src/task/strong_mode.dart
index 420fd61c972aa62d41a511067e458889fac32c52..ea4e0b349010cacd68764464160adda7a6a8c636 100644
--- a/pkg/analyzer/lib/src/task/strong_mode.dart
+++ b/pkg/analyzer/lib/src/task/strong_mode.dart
@@ -279,6 +279,12 @@ class InstanceMemberInferrer {
     List<FunctionType> overriddenTypes = new List<FunctionType>();
     for (ExecutableElement overriddenMethod in overriddenMethods) {
       FunctionType overriddenType = overriddenMethod.type;
+      if (overriddenType == null) {
+        // TODO(brianwilkerson) I think the overridden method should always have
+        // a type, but there appears to be a bug that causes it to sometimes be
+        // null, we guard against that case by not performing inference.
+        return;
+      }
       if (overriddenType.typeFormals.isNotEmpty) {
         if (overriddenType.typeFormals.length != typeFormals.length) {
           return;
@@ -466,6 +472,7 @@ class VariableGatherer extends RecursiveAstVisitor {
         }
         return element;
       }
+
       Element element = nonAccessor(node.staticElement);
       if (element is VariableElement && (filter == null || filter(element))) {
         results.add(element);