Attempt 2. The first fix was much faster but ignored syntax errors.

benchmark                      old ns/op    new ns/op    delta
BenchmarkCodeEncoder            74993543     72982390   -2.68%
BenchmarkCodeMarshal            77811181     75610637   -2.83%
BenchmarkCodeDecoder           213337123    190144982  -10.87%
BenchmarkCodeUnmarshal         212180972    190394852  -10.27%
BenchmarkCodeUnmarshalReuse    202113428    182106660   -9.90%
BenchmarkUnmarshalString            1343          919  -31.57%
BenchmarkUnmarshalFloat64           1149          908  -20.97%
BenchmarkUnmarshalInt64              967          778  -19.54%
BenchmarkSkipValue              28851581     28414125   -1.52%

benchmark                       old MB/s     new MB/s  speedup
BenchmarkCodeEncoder               25.88        26.59    1.03x
BenchmarkCodeMarshal               24.94        25.66    1.03x
BenchmarkCodeDecoder                9.10        10.21    1.12x
BenchmarkCodeUnmarshal              9.15        10.19    1.11x
BenchmarkSkipValue                 69.05        70.11    1.02x

Fixes #3949.

R=rsc
CC=golang-dev
https://golang.org/cl/7231058