Mark [this] as exposed if generative constructor contains function expression.

sdk/lib/_internal/compiler/implementation/inferrer/simple_types_inferrer.dart

Index: sdk/lib/_internal/compiler/implementation/inferrer/simple_types_inferrer.dart
diff --git a/sdk/lib/_internal/compiler/implementation/inferrer/simple_types_inferrer.dart b/sdk/lib/_internal/compiler/implementation/inferrer/simple_types_inferrer.dart
index 758d6d4004dbe2064ddfc00769bd9443cf7672ed..41914729f246c3a1bd40978298c09e1b8ddce486 100644
--- a/sdk/lib/_internal/compiler/implementation/inferrer/simple_types_inferrer.dart
+++ b/sdk/lib/_internal/compiler/implementation/inferrer/simple_types_inferrer.dart
@@ -596,6 +596,11 @@ class SimpleTypeInferrerVisitor<T>
   }
 
   T visitFunctionExpression(ast.FunctionExpression node) {
+    // We loose track of [this] in closures (see issue 20840). To be on
+    // the safe side, we mark [this] as exposed here. We could do better by
+    // analyzing the closure.
+    // TODO(herhut): Analyze whether closure exposes this.
+    isThisExposed = true;
     LocalFunctionElement element = elements.getFunctionDefinition(node);
     // We don't put the closure in the work queue of the
     // inferrer, because it will share information with its enclosing