unicode/utf16: add explicit test for decoding invalid runes.
The EncodeRune test exercises DecodeRune, but only for runes that it can encode. Add an explicit test for invalid utf16 surrogate pairs.
Bonus: coverage is now 100%
unicode/utf16/utf16.go: IsSurrogate 100.0%
unicode/utf16/utf16.go: DecodeRune 100.0%
unicode/utf16/utf16.go: EncodeRune 100.0%
unicode/utf16/utf16.go: Encode 100.0%
unicode/utf16/utf16.go: Decode 100.0%
total: (statements) 100.0%
R=golang-dev, r
CC=golang-dev
https://golang.org/cl/
39150044