Always store the type of a conditional expression on the node.

lib/ast.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// -----------------------------------------------------------------------
 ///                           ERROR HANDLING
 /// -----------------------------------------------------------------------
 ///
 /// As a rule of thumb, errors that can be detected statically are handled by
 /// the frontend, typically by translating the erroneous code into a 'throw' or
@@ skipping 1960 matching lines @@
     }
   }
 }
 
 /// Expression of form `x ? y : z`.
 class ConditionalExpression extends Expression {
   Expression condition;
   Expression then;
   Expression otherwise;
 
-  /// The static type of the expression, or `null` if the type should be derived
-  /// from [then] and [otherwise] using a very simple upper bound computation.
-  ///
-  /// If one of the arms is a null literal, the static type of the other
-  /// arm will be used.
+  /// The static type of the expression. Should not be `null`.
   DartType staticType;
 
   ConditionalExpression(this.condition, this.then, this.otherwise,
-      [this.staticType]) {
+      this.staticType) {
     condition?.parent = this;
     then?.parent = this;
     otherwise?.parent = this;
   }
 
-  DartType getStaticType(TypeEnvironment types) {
-    if (staticType != null) return staticType;
-    var left = then.getStaticType(types);
-    var right = otherwise.getStaticType(types);
-    if (left is BottomType) return right;
-    if (right is BottomType) return left;
-    return const DynamicType();
-  }
+  DartType getStaticType(TypeEnvironment types) => staticType;
 
   accept(ExpressionVisitor v) => v.visitConditionalExpression(this);
 
   visitChildren(Visitor v) {
     condition?.accept(v);
     then?.accept(v);
     otherwise?.accept(v);
     staticType?.accept(v);
   }
 
@@ skipping 1599 matching lines @@
 
   @override
   defaultTreeNode(TreeNode node) {
     if (node == child) {
       return replacement;
     } else {
       return node;
     }
   }
 }