Implement the spec's notion of LUB for function types.

pkg/analyzer/lib/dart/element/type.dart

Index: pkg/analyzer/lib/dart/element/type.dart
diff --git a/pkg/analyzer/lib/dart/element/type.dart b/pkg/analyzer/lib/dart/element/type.dart
index 5b92819fd23e6cbc89f8097b2efe35016677e537..9f079a951a6b0e1071a54e4b808c5f5a921b0022 100644
--- a/pkg/analyzer/lib/dart/element/type.dart
+++ b/pkg/analyzer/lib/dart/element/type.dart
@@ -173,6 +173,12 @@ abstract class FunctionType implements ParameterizedType {
   List<DartType> get normalParameterTypes;
 
   /**
+   * The names of the required positional parameters of this type of function,
+   * in the order that the parameters appear.
+   */
+  List<String> get normalParameterNames;
+
+  /**
    * Return a map from the names of optional (positional) parameters to the
    * types of the optional parameters of this type of function. The entries in
    * the map will be iterated in the same order as the order in which the
@@ -182,6 +188,12 @@ abstract class FunctionType implements ParameterizedType {
   List<DartType> get optionalParameterTypes;
 
   /**
+   * The names of the optional positional parameters of this type of function,
+   * in the order that the parameters appear.
+   */
+  List<String> get optionalParameterNames;
+
+  /**
    * Return a list containing the parameters elements of this type of function.
    * The parameter types are in the same order as they appear in the declaration
    * of the function.