fix #26552, improve null coalescing inference

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 analyzer.src.generated.resolver;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 6044 matching lines @@
           rightOperand.accept(this);
         } finally {
           _overrideManager.exitScope();
         }
       }
     } else {
       // TODO(leafp): Do downwards inference using the declared type
       // of the binary operator for other cases.
       if (operatorType == TokenType.QUESTION_QUESTION) {
         InferenceContext.setTypeFromNode(leftOperand, node);
-        InferenceContext.setTypeFromNode(rightOperand, node);
       }
       leftOperand?.accept(this);
+      if (operatorType == TokenType.QUESTION_QUESTION) {
+        // Set the right side, either from the context, or using the information
+        // from the left side if it is more precise.
+        DartType contextType = InferenceContext.getType(node);
+        DartType leftType = leftOperand?.staticType;
+        if (contextType == null || contextType.isDynamic) {
+          contextType = leftType;
+        }
+        InferenceContext.setType(rightOperand, contextType);
+      }
       rightOperand?.accept(this);
     }
     node.accept(elementResolver);
     node.accept(typeAnalyzer);
     return null;
   }
 
   @override
   Object visitBlockFunctionBody(BlockFunctionBody node) {
     _overrideManager.enterScope();
@@ skipping 862 matching lines @@
         node.initializer != null) {
       (element as ConstVariableElement).constantInitializer =
           new ConstantAstCloner().cloneNode(node.initializer);
     }
     return null;
   }
 
   @override
   visitVariableDeclarationList(VariableDeclarationList node) {
     for (VariableDeclaration decl in node.variables) {
+      // TODO(jmesserly): this sets the type even when it's implicitly dynamic.
+      // Do we need to do thiat?

[Leaf, 2016/08/04 22:26:44]

Might even be worth filtering out dynamic in setType.

[Jennifer Messerly, 2016/08/04 22:57:49]

Oh! good idea. I'll move the TODO and do it in a follow up. (I think it changes
some of our "inferred type here" markings. It actually seems to expose a bug
where we are doing inference and not marking. But we didn't notice because the
downwards "dynamic" counts as inference...)

       InferenceContext.setType(decl, decl.element?.type);
     }
     super.visitVariableDeclarationList(node);
   }
 
   @override
   Object visitWhileStatement(WhileStatement node) {
     // Note: since we don't call the base class, we have to maintain
     // _implicitLabelScope ourselves.
     ImplicitLabelScope outerImplicitScope = _implicitLabelScope;
@@ skipping 4034 matching lines @@
       return null;
     }
     if (identical(node.staticElement, variable)) {
       if (node.inSetterContext()) {
         result = true;
       }
     }
     return null;
   }
 }