Front-end handling of optional `new` and `const`.

[lrhn]

This is the Front-end specific issue tracking #30921.

The front-end needs to handle the new syntax and provide kernel code that treats constructor invocations equivalently whether they use the new prefix or not, and code that generates the same constant values whether nested const prefixes are used.

[peter-ahe-google]

We will not start implementation of this feature until the front-end is on par with the VM in terms of test passes.

[kmillikin]

We have an initial implementation of this feature but it is not yet complete. We fail on the following tests in the language_2 suite for various reasons (most of them seem due to const contexts):

implicit_creation/implicit_const_context_constructor_generic_named_test: CompileTimeError
implicit_creation/implicit_const_context_constructor_generic_test: CompileTimeError
implicit_creation/implicit_const_context_constructor_named_test: CompileTimeError
implicit_creation/implicit_const_context_constructor_test: CompileTimeError
implicit_creation/implicit_const_context_list_test: CompileTimeError
implicit_creation/implicit_const_context_map_test: CompileTimeError
implicit_creation/implicit_const_context_prefix_constructor_generic_named_test: CompileTimeError
implicit_creation/implicit_const_context_prefix_constructor_generic_test: CompileTimeError
implicit_creation/implicit_const_context_prefix_constructor_named_test: CompileTimeError
implicit_creation/implicit_const_context_prefix_constructor_test: CompileTimeError
implicit_creation/implicit_new_constructor_generic_named_test: CompileTimeError
implicit_creation/implicit_new_or_const_generic_test: CompileTimeError
implicit_creation/implicit_new_prefix_constructor_generic_named_test: CompileTimeError

[peter-ahe-google]

Change 41261 should fix the remaining compile-time errors.

[peter-ahe-google]

Along with change 41260 which modifies the tests to not rely on metadata on local functions. Metadata on local functions has never been implemented in dart2js, so we're not deeming that important right now. I've filed #32193 to track this.