Small refactorings in tool_runner.dart

[srawlins]

In debugging #2344, I went down some dead ends but ended up with some refactorings along the way. Some of these may be my personal preference, so are certainly up for discussion.

• the environment variables in the callback passed to config.tools.runner.run( were quite indented; extracted to a private method.
• ToolTempFileTracker had a weird or outdated doc comment; I removed it.
• ToolTempFileTracker.createTemporaryFile did not need to be async; fixed.
• Add a doc comment to ToolRunner._runProcess; edit doc comment to ToolRunner.run.
• Make a number of parameters named; in particular, toolErrorCallback is typically given a function literal, so the name helps identify what this function is supposed to be.
• Extract the code that writes the tool content to a file, into a method.

[coveralls]

Coverage increased (+0.03%) to 58.089% when pulling 4a763ae on srawlins:tidy-tool-runner into dedab6e on dart-lang:master.

[jcollins-g]

lgtm, see comment about ignore though