*[Edit by @acdlite: Decided in comments below that we will change the semantics so that a missing `fallback` prop means the exception should propagate to the next parent (like a rethrow). That way a Suspense component can specify other props like `maxDuration` without needing to provide a fallback, too.]*

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**Do you want to request a *feature* or report a *bug*?**
Bug
**What is the current behavior?**
`<React.Suspense>` does not warn you if you omit a `fallback`). While redundant for TS/Flow usage,  I misspelled the `fallback` prop by accident in a playground and was tearing my hair out trying to figure out why things were not working as expected. 

**If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem. Your bug will get fixed much faster if we can run your code and it doesn't have dependencies other than React. Paste the link to your JSFiddle (https://jsfiddle.net/Luktwrdm/) or CodeSandbox (https://codesandbox.io/s/new) example below:**

Omit a `fallback`. 

**What is the expected behavior?**
React should warn during development if `fallback` is `undefined`.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**

Master 

return (
            <Fragment>
              <Suspense
                maxDuration={1000}
                fallback={<Text text="Loading..." />}>
                <ConcurrentMode>
                  <UpdatingText ref={text1} initialText="Async: 1">
                    {text => (
                      <Fragment>
                        <Text text="Before" />
                        <AsyncText text={text} />
                        <Text text="After" />
                      </Fragment>
                    )}
                  </UpdatingText>
                </ConcurrentMode>
              </Suspense>
              <ConcurrentMode>
                <UpdatingText ref={text2} initialText="Sync: 1">
                  {text => (
                    <Fragment>
                      <Text text="Before" />
                      <Text text={text} />
                      <Text text="After" />
                    </Fragment>
                  )}
                </UpdatingText>
              </ConcurrentMode>
            </Fragment>