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Target-specific modular transformations have to be able to cope with external libraries. Target-specific global transformations should be optimizations, and not required for correctness, since a simple linker may choose not to perform them. Make mixin resolution modular by making it fail when a mixed-in class comes from an external library. (We cannot mix in such a class because we do not necessarily have all class members.) R=asgerf@google.com Review-Url: https://codereview.chromium.org/2671653003 .