Pragma for testing multiple entrypoints

Example:

void hook(String functionName, int entryPointId) {
  // ...
}

class C<T> {
  @pragma("vm:testing.unsafe.trace-entrypoints-fn", hook)
  void foo(T x) {
    // ...
  }
}

When foo is invoked, hook will be called in foo's prologue if foo was compiled with multiple entrypoints. hook will be passed the name of the function it was called for and the ID of the entrypoint used for the invocation:

• 0: Normal entry.

• 1: Unchecked entry: prologue was short so separate prologues for normal and unchecked entry were compiled.

• 2: Unchecked shared entry: prologue was long, so normal and unchecked entry set a temporary and type-checks are predicated on the temporary.