[dart/vm] add ability to pass VM flags to dart when running gen_kernel.dart in precompiler2 script

Rationale:
The objective is to use
  dart --enable-asserts gen_kernel.dart ....
using DART_VM_FLAGS

#38965

Change-Id: Ibb255b51fa956d841103e85a0129dfc9974b4e11
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/122165
Commit-Queue: Aart Bik <ajcbik@google.com>
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Daco Harkes <dacoharkes@google.com>

Author: aartbik

pkg/vm/tool/precompiler2

@@ -92,6 +92,7 @@ BIN_DIR="$OUT_DIR/$DART_CONFIGURATION"
 
 # Step 1: Generate Kernel binary from the input Dart source.
 "$BIN_DIR"/dart                                                                \
+     ${DART_VM_FLAGS}                                                          \
      "${SDK_DIR}/pkg/vm/bin/gen_kernel.dart"                                   \
      --platform "${BIN_DIR}/vm_platform_strong.dill"                           \
      --aot                                                                     \