fix #26896, mark all @proxy ops as dynamic

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

Index: pkg/analyzer/lib/src/task/strong/checker.dart
diff --git a/pkg/analyzer/lib/src/task/strong/checker.dart b/pkg/analyzer/lib/src/task/strong/checker.dart
index 19e1c8c02bbc57e12372d140195c568a86a416b5..85957f6bd9c5e98c73928f43fe4f6d41e67faadf 100644
--- a/pkg/analyzer/lib/src/task/strong/checker.dart
+++ b/pkg/analyzer/lib/src/task/strong/checker.dart
@@ -1029,7 +1029,10 @@ class CodeChecker extends RecursiveAstVisitor {
     // Null type happens when we have unknown identifiers, like a dart: import
     // that doesn't resolve.
     var type = node.staticType;
-    return type == null || type.isDynamic;
+    var element = type?.element;
+    return type == null ||
+        type.isDynamic ||
+        (element is ClassElement && element.isOrInheritsProxy);

[vsm, 2016/08/09 15:20:51]

Is this too conservative if there is an actual method on the class?

E.g., foo vs bar below:

@Proxy()
class A {
  void foo(int x) { ... }
  noSuchMethod(...) { ... }
}

var a = new A();
a.foo(42); // Not dynamic
a.bar(42); // Dynamic

[Jennifer Messerly, 2016/08/09 15:31:11]

This path actually only handles operators. Method/function calls were already
handled by _isDynamicCall, and they use the absence of an Element to figure out
that it must be a dynamic call.

For operators we might be able to use a similar technique. It would mean
reworking _isDynamicTarget, removing it and treating operators as dynamic calls.
Not sure how well that would work but it is the only approach I think that would
address your concern here. But you're right, we're a tad conservative on using
operators on @proxy classes.

   }
 
   bool _isLibraryPrefix(Expression node) =>