It is an error if a type name appears multiple times in an implements clause

[bwilkerson]

The specification disallows code of the following form:

class A implement I, J, I {}

"It is a compile-time error if a type T appears more than once in the implements clause of a class."

See http://code.google.com/p/dart/issues/detail?id=976 for more details.

[bwilkerson]

Added this to the M2 milestone.

[scheglov]

Set owner to @scheglov.
Removed this from the M2 milestone.
Added this to the M1 milestone.
Added Accepted label.

[scheglov]

https://codereview.chromium.org/10918247/

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Added Started label.

[scheglov]

https://code.google.com/p/dart/source/detail?r=12391

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Added Fixed label.