Fix bot redness.

sdk/lib/_internal/compiler/implementation/types/concrete_types_inferrer.dart

Index: sdk/lib/_internal/compiler/implementation/types/concrete_types_inferrer.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/types/concrete_types_inferrer.dart	(revision 25147)
+++ sdk/lib/_internal/compiler/implementation/types/concrete_types_inferrer.dart	(working copy)
@@ -1365,7 +1365,7 @@
     });
 
     // handle initializing formals
-    element.functionSignature.forEachParameter((param) {
+    element.computeSignature(compiler).forEachParameter((param) {
       if (param.kind == ElementKind.FIELD_PARAMETER) {
         FieldParameterElement fieldParam = param;
         augmentFieldType(fieldParam.fieldElement,
@@ -1383,7 +1383,7 @@
         new TypeInferrerVisitor(elements, element, this, environment);
 
     bool foundSuperOrRedirect = false;
-    if (tree.initializers != null) {
+    if (tree != null && tree.initializers != null) {
       // we look for a possible call to super in the initializer list
       for (final init in tree.initializers) {
         init.accept(visitor);
@@ -1416,7 +1416,7 @@
       }
     }
 
-    tree.accept(visitor);
+    if (tree != null) tree.accept(visitor);
     return singletonConcreteType(new ClassBaseType(enclosingClass));
   }