Dart_NewStringFromUTF16 and Dart_NewStringFromUTF32 should check for valid input

[a-siva]

Dart_NewStringFromUTF16 should ensure that the input utf16_array
consists of valid values similar to how Dart_NewStringFromUTF8 validates
the input.

[lexprfuncall]

The UTF-16 validity check should exclude unpaired lead and trail surrogates. The UTF-32 validity check should reject these values outright, whether paired or not.

There are other classes of characters which might be worthwhile excluding. However, this seems like a reasonable starting point.

[iposva-google]

Removed Priority-Medium label.
Added Priority-Unassigned label.