Refactor least upper bound computation; support bottom correctly.

pkg/analyzer/test/generated/static_type_warning_code_test.dart

Index: pkg/analyzer/test/generated/static_type_warning_code_test.dart
diff --git a/pkg/analyzer/test/generated/static_type_warning_code_test.dart b/pkg/analyzer/test/generated/static_type_warning_code_test.dart
index 486d52ef2298b768f4d12883a9c9e3f98e1c2878..05a6b993f09d345bfa5f60f11a7844aefed567d9 100644
--- a/pkg/analyzer/test/generated/static_type_warning_code_test.dart
+++ b/pkg/analyzer/test/generated/static_type_warning_code_test.dart
@@ -1567,6 +1567,12 @@ class C {
     assertErrors(source, [StaticTypeWarningCode.UNDEFINED_METHOD]);
   }
 
+  void test_undefinedMethod_leastUpperBoundWithNull() {
+    Source source = addSource('f(bool b, int i) => (b ? null : i).foo();');
+    computeLibrarySourceErrors(source);
+    assertErrors(source, [StaticTypeWarningCode.UNDEFINED_METHOD]);
+  }
+
   void test_undefinedMethod_object_call() {
     AnalysisOptionsImpl options = new AnalysisOptionsImpl();
     options.enableStrictCallChecks = true;