Add the ability to create objects through the C API

[sgjesse]

It is not possible to create new Dart object instances from the C API.

A function like this is proposed for the API:

Dart_Handle Dart_InvokeConstuctor(Dart_Handle library,
                                  Dart_Handle class_name,
                                  Dart_Handle constructor_name,
                                  int number_of_arguments,
                                  Dart_Handle* arguments);

The class_name could also be the class object retrieved through Dart_GetClass.

[turnidge]

Set owner to @turnidge.
Added Accepted label.

[turnidge]

The function Dart_New has been added to the api:

/**
 * Invokes a constructor, creating a new object.
 *
 * This function allows hidden constructors (constructors with leading
 * underscores) to be called.
 *
 * \param clazz A class or an interface.
 * \param constructor_name The name of the constructor to invoke. Use
 * Dart_Null() to invoke the unnamed constructor. This name should
 * not include the name of the class.
 * \param number_of_arguments Size of the arguments array.
 * \param arguments An array of arguments to the constructor.
 *
 * \return If the constructor is called and completes successfully,
 * then the new object. If an error occurs during execution, then an
 * error handle is returned.
 /
DART_EXPORT Dart_Handle Dart_New(Dart_Handle clazz,
                                 Dart_Handle constructor_name,
                                 int number_of_arguments,
                                 Dart_Handle arguments);

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Added Fixed label.