Issue 23409. Don't set 'propagatedType' in (vs. after) 'name is Type'.

pkg/analyzer/lib/src/generated/resolver.dart

 // Copyright (c) 2014, 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.
 
 library engine.resolver;
 
 import "dart:math" as math;
 import 'dart:collection';
 
 import 'package:analyzer/src/generated/utilities_collection.dart';
@@ skipping 10491 matching lines @@
   /**
    * If it is appropriate to do so, override the current type of the static and propagated elements
    * associated with the given expression with the given type. Generally speaking, it is appropriate
    * if the given type is more specific than the current type.
    *
    * @param expression the expression used to access the static and propagated elements whose types
    *          might be overridden
    * @param potentialType the potential type of the elements
    * @param allowPrecisionLoss see @{code overrideVariable} docs
    */
-  void overrideExpression(
-      Expression expression, DartType potentialType, bool allowPrecisionLoss) {
+  void overrideExpression(Expression expression, DartType potentialType,
+      bool allowPrecisionLoss, bool setExpressionType) {
     VariableElement element = getOverridableStaticElement(expression);
     if (element != null) {
       DartType newBestType =
           overrideVariable(element, potentialType, allowPrecisionLoss);
-      recordPropagatedTypeIfBetter(expression, newBestType);
+      if (setExpressionType) {
+        recordPropagatedTypeIfBetter(expression, newBestType);
+      }
     }
     element = getOverridablePropagatedElement(expression);
     if (element != null) {
       overrideVariable(element, potentialType, allowPrecisionLoss);
     }
   }
 
   /**
    * If it is appropriate to do so, override the current type of the given element with the given
    * type.
@@ skipping 102 matching lines @@
     }
     return super.visitAnnotation(node);
   }
 
   @override
   Object visitAsExpression(AsExpression node) {
     super.visitAsExpression(node);
     // Since an as-statement doesn't actually change the type, we don't
     // let it affect the propagated type when it would result in a loss
     // of precision.
-    overrideExpression(node.expression, node.type.type, false);
+    overrideExpression(node.expression, node.type.type, false, false);
     return null;
   }
 
   @override
   Object visitAssertStatement(AssertStatement node) {
     super.visitAssertStatement(node);
     _propagateTrueState(node.condition);
     return null;
   }
 
@@ skipping 1044 matching lines @@
       if (binary.operator.type == sc.TokenType.BAR_BAR) {
         _propagateFalseState(binary.leftOperand);
         _propagateFalseState(binary.rightOperand);
       }
     } else if (condition is IsExpression) {
       IsExpression is2 = condition;
       if (is2.notOperator != null) {
         // Since an is-statement doesn't actually change the type, we don't
         // let it affect the propagated type when it would result in a loss
         // of precision.
-        overrideExpression(is2.expression, is2.type.type, false);
+        overrideExpression(is2.expression, is2.type.type, false, false);
       }
     } else if (condition is PrefixExpression) {
       PrefixExpression prefix = condition;
       if (prefix.operator.type == sc.TokenType.BANG) {
         _propagateTrueState(prefix.operand);
       }
     } else if (condition is ParenthesizedExpression) {
       _propagateFalseState(condition.expression);
     }
   }
@@ skipping 20 matching lines @@
       if (binary.operator.type == sc.TokenType.AMPERSAND_AMPERSAND) {
         _propagateTrueState(binary.leftOperand);
         _propagateTrueState(binary.rightOperand);
       }
     } else if (condition is IsExpression) {
       IsExpression is2 = condition;
       if (is2.notOperator == null) {
         // Since an is-statement doesn't actually change the type, we don't
         // let it affect the propagated type when it would result in a loss
         // of precision.
-        overrideExpression(is2.expression, is2.type.type, false);
+        overrideExpression(is2.expression, is2.type.type, false, false);
       }
     } else if (condition is PrefixExpression) {
       PrefixExpression prefix = condition;
       if (prefix.operator.type == sc.TokenType.BANG) {
         _propagateFalseState(prefix.operand);
       }
     } else if (condition is ParenthesizedExpression) {
       _propagateTrueState(condition.expression);
     }
   }
@@ skipping 3712 matching lines @@
     nonFields.add(node);
     return null;
   }
 
   @override
   Object visitNode(AstNode node) => node.accept(TypeResolverVisitor_this);
 
   @override
   Object visitWithClause(WithClause node) => null;
 }