Frog fails to treat generic factory methods as true generic methods with a type scope

[DartBot]

This issue was originally filed by jimhug@google.com

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This leads to two warnings when compiling frog with itself.

leg/util/link_implementation.dart:10:27: warning: cannot find type T
      tail = new LinkTail<T>(); // Frog bug: T is in scope.
                          ^
leg/util/link_implementation.dart:13:26: warning: cannot find type T
    return new LinkEntry<T>(head, tail); // Frog bug: T is in scope.
                         ^

Swallowing my annoyance and just implementing the damn feature...

[DartBot]

This comment was originally written by jimhug@google.com

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http://codereview.chromium.org/8681027/

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Added Fixed label.