Fix an exception thrown when a method cannot be resolved because the target type is dynamic

Change-Id: I0479305057a2f763f68aa61c860ab9785d2ca1b9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/151420
Reviewed-by: Jaime Wren <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>

Author: bwilkerson

pkg/analysis_server/lib/src/services/completion/dart/feature_computer.dart

@@ -720,9 +720,15 @@ extension AstNodeFeatureComputerExtension on AstNode {
   /// Return the [FunctionType], if there is one, for this [AstNode].
   FunctionType get functionType {
     if (parent is MethodInvocation) {
-      return (parent as MethodInvocation).staticInvokeType;
+      var type = (parent as MethodInvocation).staticInvokeType;
+      if (type is FunctionType) {
+        return type;
+      }
     } else if (parent is FunctionExpressionInvocation) {
-      return (parent as FunctionExpressionInvocation).staticInvokeType;
+      var type = (parent as FunctionExpressionInvocation).staticInvokeType;
+      if (type is FunctionType) {
+        return type;
+      }
     }
     return null;
   }