Check for non-null assignment when checking casts

pkg/analyzer/lib/src/task/strong/checker.dart

 // Copyright (c) 2015, 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.
 
 // TODO(jmesserly): this was ported from package:dev_compiler, and needs to be
 // refactored to fit into analyzer.
 library analyzer.src.task.strong.checker;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 178 matching lines @@
       DartType expectedType = _elementType(element);
       if (expectedType == null) expectedType = DynamicTypeImpl.instance;
       checkArgument(arg, expectedType);
     }
   }
 
   void checkAssignment(Expression expr, DartType type) {
     if (expr is ParenthesizedExpression) {
       checkAssignment(expr.expression, type);
     } else {
-      if (_checkNonNullAssignment(expr, type)) {
-        _checkDowncast(expr, type);
-      }
+      _checkDowncast(expr, type);
     }
   }
 
   /// Analyzer checks boolean conversions, but we need to check too, because
   /// it uses the default assignability rules that allow `dynamic` and `Object`
   /// to be assigned to bool with no message.
   void checkBoolean(Expression expr) =>
       checkAssignment(expr, typeProvider.boolType);
 
   void checkFunctionApplication(InvocationExpression node) {
@@ skipping 524 matching lines @@
   ///
   /// If [from] is omitted, uses the static type of [expr].
   ///
   /// If [expr] does not require a downcast because it is not related to [to]
   /// or is already a subtype of it, does nothing.
   void _checkDowncast(Expression expr, DartType to, {DartType from}) {
     if (from == null) {
       from = _getDefiniteType(expr);
     }
 
+    if (!_checkNonNullAssignment(expr, to, from)) return;
+
     // We can use anything as void.
     if (to.isVoid) return;
 
     // fromT <: toT, no coercion needed.
     if (rules.isSubtypeOf(from, to)) return;
 
     // Note: a function type is never assignable to a class per the Dart
     // spec - even if it has a compatible call method.  We disallow as
     // well for consistency.
     if (from is FunctionType && rules.getCallMethodType(to) != null) {
@@ skipping 114 matching lines @@
       // Variable declaration is inside a function or method, so it's safe.
       return;
     }
     _recordMessage(node, StrongModeCode.UNSAFE_BLOCK_CLOSURE_INFERENCE,
         [declElement.name]);
   }
 
   /// Checks if the assignment is valid with respect to non-nullable types.
   /// Returns `false` if a nullable expression is assigned to a variable of
   /// non-nullable type and `true` otherwise.
-  bool _checkNonNullAssignment(Expression expression, DartType type) {
-    var exprType = expression.staticType;
-    if (rules.isNonNullableType(type) && rules.isNullableType(exprType)) {
+  bool _checkNonNullAssignment(Expression expression, DartType to, DartType from) {
+    if (rules.isNonNullableType(to) && rules.isNullableType(from)) {
       _recordMessage(expression, StaticTypeWarningCode.INVALID_ASSIGNMENT,
-          [exprType, type]);
+          [from, to]);
       return false;
     }
     return true;
   }
 
   void _checkReturnOrYield(Expression expression, AstNode node,
       {bool yieldStar: false}) {
     FunctionBody body = node.getAncestor((n) => n is FunctionBody);
     var type = _getExpectedReturnType(body, yieldStar: yieldStar);
     if (type == null) {
@@ skipping 582 matching lines @@
     var visited = new Set<InterfaceType>();
     do {
       visited.add(current);
       current.mixins.reversed.forEach(
           (m) => _checkIndividualOverridesFromClass(node, m, seen, true));
       _checkIndividualOverridesFromClass(node, current.superclass, seen, true);
       current = current.superclass;
     } while (!current.isObject && !visited.contains(current));
   }
 }