Adjust tonic::DartInvokeField to avoid direct invocation of the field.

[sstrickl]

Previously, tonic::DartInvokeField called Dart_Invoke. Once Dart starts enforcing entry point verification, this will fail because fields cannot be annotated as invokable. Instead, first call Dart_GetField to get the closure value in the field and then invoke that using Dart_InvokeClosure.

This is needed to enable entry point verification by default, related to the following PRs:

• Support for weak entry points in the VM dart-lang/sdk#50649
• PluginUtilities.getCallbackHandle and tree-shaking flutter#118608

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