Let me provide -D "environment" variables for browser tests in the command line

For content-shell and Dartium: pipe them in at runtime
For js output: provide the to dart2js

Scenario: I'm hitting scenarios where I need to provide runtime configuration – specifically secrets – when testing on a CI server.

Start by tracing how browser-test command-line arguments are handled for content-shell, Dartium, and JavaScript output. Verify how each target receives runtime configuration, then define done as environment variables supplied with -D reaching the test process or dart2js output as requested.

Written by the indexing model from the issue text.