[dartdevc] Generate dart_sdk debug module metadata

[annagrin]

Currently dart_sdk module does not come with the metadata, causing evaluating expression referring to types and libraries from the dart_sdk module to fail. Example: evaluating a dart type, i.e String in the expression evaluation box fails due to the debugger being unaware of dart:_runtime library.

See: dart-lang/webdev#1226

Suggestion:

• Can we create metadata for sdk module, listing all implicitly imported libraries?
• Need to make sure the debugger can find it (similar to dart_sdk.js file).

Needed for:
Better implementation of: dart-lang/webdev#1457 (see discussion in dart-lang/webdev#1472)