The default mirror tries to access implementation details of the
generator, accessing the 'description' property of the storage, which is
an NSDictionary subclass. The default implementation of -[NSDictionary
description] tries to print the dictionary contents. But if the
Swift.Dictionary can't be bridged to NSDictionary, that causes a runtime
trap.
rdar://problem/24238609
This code as written is not sound and should not type check
for non-final classes conforming to SequenceType (eg, NSArray).
Instead, capture the base of the call from the preprocess closure
passed in. The closure is @noescape, so it should be equivalent.
This brings down StdlibUnittest build time to 90 seconds with either
a DebugAssert or a ReleaseAssert compiler.
The new library, StdlibCollectionTests, is only built when running
validation tests.
