Fix and assert getLocalFunction invariant

pkg/compiler/lib/src/kernel/element_map_impl.dart

Index: pkg/compiler/lib/src/kernel/element_map_impl.dart
diff --git a/pkg/compiler/lib/src/kernel/element_map_impl.dart b/pkg/compiler/lib/src/kernel/element_map_impl.dart
index c08b5f6030a563b28fe19db3d81504653bde6d24..642806795aac7108b8ac6ded81471beac1f0d485 100644
--- a/pkg/compiler/lib/src/kernel/element_map_impl.dart
+++ b/pkg/compiler/lib/src/kernel/element_map_impl.dart
@@ -896,6 +896,10 @@ abstract class ElementCreatorMixin {
   }
 
   Local _getLocalFunction(ir.TreeNode node) {
+    assert(
+        node is ir.FunctionDeclaration || node is ir.FunctionExpression,
+        failedAt(
+            CURRENT_ELEMENT_SPANNABLE, 'Invalid local function node: $node'));
     return _localFunctionMap.putIfAbsent(node, () {
       MemberEntity memberContext;
       Entity executableContext;