dart2js modular FE and analysis support

[sigmundch]

Meta issue to track tasks related to modular support in dart2js.

This includes:

• [x] support generating dart2js kernel in kernel_worker used by bazel and build runner
• [x] add support for running dart2js modular tests in the SDK
• [x] add support in dart2js to load multiple .dill files and assemble the full program in memory
• [ ] add support in dart2js to emit modular analysis data
• [ ] enable constant-update-2018, so that dart2js can use the kernel-ir by default to represent constants.
• [ ] add support to load modular analysis data files.

Not tracked here, but necessary:

• starlark and build_runner hooks to invoke the dart2js compiler modularly.

[vsmenon]

@sigmundch - anything left that you want to land in D24 (here or in the other D24 bug)?

[sigmundch]

moved to a later milestone