Implement async* functions in VM

tests/language/language.status

Index: tests/language/language.status
===================================================================
--- tests/language/language.status	(revision 43971)
+++ tests/language/language.status	(working copy)
@@ -10,7 +10,6 @@
 await_for_cancel_test: RuntimeError # Issue 21404
 asyncstar_yield_test: Fail # Issue 21404
 asyncstar_yieldstar_test: Fail # Issue 21404
-asyncstar_concat_test: Fail # Issue 21404
 async_rethrow_test: RuntimeError # Issue 22523
 
 # These bugs refer currently ongoing language discussions.
@@ -48,25 +47,9 @@
 least_upper_bound_expansive_test/*: Fail, OK
 
 
-async_await_syntax_test/a09a: CompileTimeError # Issue 21404
-async_await_syntax_test/a10a: CompileTimeError # Issue 21404
-async_await_syntax_test/b09a: CompileTimeError # Issue 21404
-async_await_syntax_test/b10a: CompileTimeError # Issue 21404
-async_await_syntax_test/c09a: CompileTimeError # Issue 21404
-async_await_syntax_test/c10a: CompileTimeError # Issue 21404
-async_await_syntax_test/d09a: CompileTimeError # Issue 21404
-async_await_syntax_test/d10a: CompileTimeError # Issue 21404
-
 [ $compiler == none || ($compiler == dart2dart && $builder_tag != new_backend) ]
 async_throw_in_catch_test: Crash # Issue 22445, 22446
 asyncstar_throw_in_catch_test: CompileTimeError # Issue 21404
-async_await_syntax_test/a03a: CompileTimeError # Issue 21404
-async_await_syntax_test/a03b: CompileTimeError # Issue 21404
-async_await_syntax_test/a11d: CompileTimeError # Issue 21404
-async_await_syntax_test/b03a: CompileTimeError # Issue 21404
-async_await_syntax_test/b11d: CompileTimeError # Issue 21404
-async_await_syntax_test/c03a: CompileTimeError # Issue 21404
-async_await_syntax_test/d03a: CompileTimeError # Issue 21404
 sync_generator3_test/test2: Fail # Issue 22300
 
 [ $compiler == none && ($runtime == drt || $runtime == dartium|| $runtime == ContentShellOnAndroid) ]