[BUILD] - Initial CL to unfork the NNBD Dart SDK

NOTE: This is a trial submit and will be reverted immediately.

- Flips the flag from --nnbd to --no-nnbd so that by default it builds
  the NNBD version
- using the --no-nnbd flag results in the SDK being built in a
  directory which has the 'Legacy' suffix added to it
  (e.g: out/DebugX64Legacy)
- the '--enable-experiment=non-nullable' flag still needs to be passed
  in during execution so that CFE runs in that mode. This is different
  from the 'null_safety' flag

Change-Id: I7d25d9710818af5919c0bb83abe51153172f5886
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/144880
Reviewed-by: Siva Annamalai <asiva@google.com>

utils/dartanalyzer/BUILD.gn

@@ -44,7 +44,7 @@ prebuilt_dart_action("generate_summary_strong") {
   inputs = sdk_lib_files + analyzer_files
   output = "$root_gen_dir/strong.sum"
   outputs = [ output ]
-  if (use_nnbd) {
+  if (!dont_use_nnbd) {
     args = [
       "build-non-nullable",
       rebase_path(output),