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Background: https://github.com/dart-lang/sdk/issues/24933 Salient bits: * sampling will only start if a client registers interest in diagnostics. * sampling will stop if clients stop requesting diagnostics. * sample results are coming back as a string (but a double would be better). * the mapping of context folders to averages in not garbage collected; since it's only potentially leaking `_Average` objects (and not contexts) when folders are deleted, I think that's fairly benign. BUG=24933 R=brianwilkerson@google.com, devoncarew@google.com Review URL: https://codereview.chromium.org/1463923003 .