json.dart doesn't escape all control characters

[DartBot]

This issue was originally filed by rice@google.com

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In json.dart, the functions _needsEscape and _escape fail to deal with control characters other than new line '\n' and carriage return '\r' (incorrectly referred to in the source code a sLINE_FEED -- which is a synonym for newline).

According to the JSON specification, all control characters (char codes between 0 and 31) must be escaped, either as \b, \f, \n, \r, \t or as \uxxxx.

[iposva-google]

Dan do you have time to fix this yourself? Thanks.

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Set owner to rice@google.com.
Added Accepted label.

[DartBot]

This comment was originally written by rice@google.com

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Will do.

[DartBot]

This comment was originally written by rice@google.com

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Added Fixed label.