[vm] Always complete Future synchronously when returning non-Future value from async functions

From Dart language specification (17.15 Function Invocation):

If execution of the body returns an object, o is completed with that
object. If it completes normally or returns without an object,
o is completed with the null object (17.4), and if it throws
an exception e and stack trace t, o is completed with the error e and
stack trace t. If execution of the body throws before the body suspends
the first time, completion of o happens at some future time after the
invocation has returned. The caller needs time to set up error handling
for the returned future, so the future is not completed with
an error before it has been returned.

So far, implementation of async functions in Dart VM postponed
completion of the Future both with the value and with an error
if execution has not reached the first await. This change modifies
the behavior to compile future with value immediately even if
the first await was not reached. Completion with an error is still
postponed.

This change in behavior will let us improve performance of
async/await in certain cases.

The new behavior still conforms to the Dart language specification.

TEST=ci
Issue: #48378

Change-Id: I0e80a1d0dd0c3cd6acbc640681e7b06bbf81c19c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/243101
Reviewed-by: Lasse Nielsen <[email protected]>
Commit-Queue: Alexander Markov <[email protected]>

Author: alexmarkov

sdk/lib/_internal/vm/lib/async_patch.dart

@@ -392,14 +392,16 @@ class _SuspendState {
           'returnValue=$returnValue)');
     }
     _Future future;
-    bool isSync = true;
     if (suspendState is _SuspendState) {
       future = unsafeCast<_Future>(suspendState._functionData);
     } else {
       future = unsafeCast<_Future>(suspendState);
-      isSync = false;
     }
-    _completeOnAsyncReturn(future, returnValue, isSync);
+    if (returnValue is Future) {
+      future._asyncCompleteUnchecked(returnValue);
+    } else {
+      future._completeWithValue(returnValue);
+    }
     return future;
   }
 
@@ -412,14 +414,12 @@ class _SuspendState {
           'returnValue=$returnValue)');
     }
     _Future future;
-    bool isSync = true;
     if (suspendState is _SuspendState) {
       future = unsafeCast<_Future>(suspendState._functionData);
     } else {
       future = unsafeCast<_Future>(suspendState);
-      isSync = false;
     }
-    _completeWithNoFutureOnAsyncReturn(future, returnValue, isSync);
+    future._completeWithValue(returnValue);
     return future;
   }