[Flight] Emit start time before an await if one wasn't emitted already

[sebmarkbage]

There's a special case where if we create a new task, e.g. to serialize a promise like <div>{promise}</div> then that row doesn't have any start time emitted but it has a task.time inherited. We mostly don't need this because every other operation emits its own start time. E.g. when we started rendering a Server Component or the real start time of a real await.

For these implied awaits we don't have a start time. Ideally it would probably be when we started the serialization, like when we called .then() but we can't just emit that eagerly and we can't just advance the task.time because that time represents the last render or previous await and we use that to cut off awaits. However for this case we don't want to cut off any inner awaits inside the node we're serializing if they happened before the .then().

Therefore, I just use the time of the previous operation - which is likely either the resolution of a previous promise that blocked the <div> like the promise of the Server Component that rendered it, or just the start of the Server Component if it was sync.

[react-sizebot]

Comparing: 9b2a545...0a51593

Critical size changes

Includes critical production bundles, as well as any change greater than 2%:

Name	+/-	Base	Current	+/- gzip	Base gzip	Current gzip
oss-stable/react-dom/cjs/react-dom.production.js	=	6.68 kB	6.68 kB	=	1.83 kB	1.83 kB
oss-stable/react-dom/cjs/react-dom-client.production.js	=	530.57 kB	530.57 kB	=	93.67 kB	93.67 kB
oss-experimental/react-dom/cjs/react-dom.production.js	=	6.69 kB	6.69 kB	=	1.83 kB	1.83 kB
oss-experimental/react-dom/cjs/react-dom-client.production.js	=	651.66 kB	651.66 kB	=	114.79 kB	114.79 kB
facebook-www/ReactDOM-prod.classic.js	=	674.81 kB	674.81 kB	=	118.78 kB	118.78 kB
facebook-www/ReactDOM-prod.modern.js	=	665.30 kB	665.30 kB	=	117.19 kB	117.19 kB

Significant size changes

Includes any change greater than 0.2%:

(No significant changes)

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