[VM] Relax assertions in NativeArguments allowing native generic functions

NativeArguments don't allow you to access type arguments, but calling
conventions are setup in such a way that nothing else needs to be 
changed.

This is needed because we changed

List makeListFixedLength(List growableList)
    native "Internal_makeListFixedLength";

to 

List<T> makeListFixedLength<T>(List<T> growableList)
    native "Internal_makeListFixedLength";

Bug:
Change-Id: I2f8755f25c488a422a60f3efce2541de7cbeae0e
Reviewed-on: https://dart-review.googlesource.com/15080
Reviewed-by: Vyacheslav Egorov <vegorov@google.com>

runtime/vm/native_arguments.h

@@ -151,7 +151,6 @@ class NativeArguments {
 
   static intptr_t ParameterCountForResolution(const Function& function) {
     ASSERT(function.is_native());
-    ASSERT(!function.IsGeneric());                // Not supported.
     ASSERT(!function.IsGenerativeConstructor());  // Not supported.
     intptr_t count = function.NumParameters();
     if (function.is_static() && function.IsClosureFunction()) {
@@ -166,7 +165,6 @@ class NativeArguments {
 
   static int ComputeArgcTag(const Function& function) {
     ASSERT(function.is_native());
-    ASSERT(!function.IsGeneric());                // Not supported.
     ASSERT(!function.IsGenerativeConstructor());  // Not supported.
     int tag = ArgcBits::encode(function.NumParameters());
     int function_bits = 0;