[vm/corelib] Make sure _Closure.call is not tree-shaken

_Closure.call is implicitly used in the VM to implement function calls.
It should never be tree-shaken even if there are no explicit calls to
_Closure.call.

TEST=Manually patched in https://dart-review.googlesource.com/c/sdk/+/265962
which removes remaining dynamic calls to 'call' from core library
and verified that 'Hello world' works in AOT mode.

Change-Id: Ifd6803c98892f38e037abb0aa0ea1eb810621a58
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/266423
Commit-Queue: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Slava Egorov <vegorov@google.com>

sdk/lib/_internal/vm/lib/function.dart

@@ -18,6 +18,7 @@ class _Closure implements Function {
     return _hash;
   }
 
+  @pragma("vm:entry-point")
   _Closure get call => this;
 
   @pragma("vm:external-name", "Closure_computeHash")