Infer VariableSet types.

pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart

 // Copyright (c) 2017, 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.
 
 /// This file declares a "shadow hierarchy" of concrete classes which extend
 /// the kernel class hierarchy, adding methods and fields needed by the
 /// BodyBuilder.
 ///
 /// Instances of these classes may be created using the factory methods in
 /// `ast_factory.dart`.
@@ skipping 866 matching lines @@
       // KernelVariableDeclaration.
       mutatedInClosure = true;
       declaredType = variable.type;
     }
     return inferrer.inferVariableGet(typeContext, typeNeeded, mutatedInClosure,
         _fact, _scope, fileOffset, declaredType, (type) {
       promotedType = type;
     });
   }
 }
+
+/// Concrete shadow object representing a write to a variable in kernel form.
+class KernelVariableSet extends VariableSet implements KernelExpression {
+  KernelVariableSet(VariableDeclaration variable, Expression value)
+      : super(variable, value);
+
+  @override
+  DartType _inferExpression(
+      KernelTypeInferrer inferrer, DartType typeContext, bool typeNeeded) {
+    DartType declaredType;
+    var variable = this.variable;
+    if (variable is KernelVariableDeclaration) {
+      declaredType = variable._declaredType;
+    } else {
+      // Hack to deal with the fact that BodyBuilder still creates raw
+      // VariableDeclaration objects sometimes.
+      // TODO(paulberry): get rid of this once the type parameter is
+      // KernelVariableDeclaration.
+      declaredType = variable.type;
+    }
+    // TODO(scheglov) Should we use the declared type, or the promoted type?

[Paul Berry, 2017/05/04 19:31:03]

I believe the declared type is better, since the purpose of the promoted type is
to capture type information that is known about the value stored in the
variable.  Since a VariableSet obliterates the value stored in the variable, the
way we type infer it shouldn't be affected by what was previously stored in it.

[scheglov, 2017/05/04 19:41:01]

Acknowledged.

+    return inferrer.inferVariableSet(typeNeeded, declaredType, value);

[scheglov, 2017/05/04 18:41:58]

We don't need "typeContext" in inferrer, should we still pass it in for
consistency?

[Paul Berry, 2017/05/04 19:31:03]

I would prefer for us to pass it in for consistency with inferBoolLiteral,
inferDoubleLiteral, etc.  

[scheglov, 2017/05/04 19:41:01]

Done.

+  }
+}