Prior to this change, the `setClassMetadata` call would be invoked
inside of an IIFE that was marked as pure. This allows the call to be
tree-shaken away in production builds, as the `setClassMetadata` call
is only present to make the original class metadata available to the
testing infrastructure. The pure marker is problematic, though, as the
`setClassMetadata` call does in fact have the side-effect of assigning
the metadata into class properties. This has worked under the assumption
that only build optimization tools perform tree-shaking, however modern
bundlers are also able to elide calls that have been marked pure so this
assumption does no longer hold. Instead, an `ngDevMode` guard is used
which still allows the call to be elided but only by tooling that is
configured to consider `ngDevMode` as constant `false` value.
PR Close#39987
1f73af77a7