fix #26552, improve null coalescing inference

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

Index: pkg/analyzer/lib/src/generated/resolver.dart
diff --git a/pkg/analyzer/lib/src/generated/resolver.dart b/pkg/analyzer/lib/src/generated/resolver.dart
index afa7ccec98ee9c48746663cc1c1a9893c0b0b541..7b9571b61f4e50aa1ccb027b5389d39b68a8f6ee 100644
--- a/pkg/analyzer/lib/src/generated/resolver.dart
+++ b/pkg/analyzer/lib/src/generated/resolver.dart
@@ -6062,9 +6062,18 @@ class ResolverVisitor extends ScopedVisitor {
       // 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);
@@ -6947,6 +6956,8 @@ class ResolverVisitor extends ScopedVisitor {
   @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);