The current implementation only emits APIs that are on the
standards track and supported in Chrome, Firefox, and Safari.
This leaves out widely used APIs like Trusted Types, so this
change relaxes those requirements. In order to support this
change, a number of changes are included:

- BrowserCompatData is modified to handle some slight
discrepancies in how compatibility data is stored, including
global APIs, namespaces, static members, and event handlers.
- Interfaces and namespaces are generated based on whether they
are standards track and experimental. If they are not generated,
any references to them will be replaced by the equivalent JS
type.
- Likewise, inheritance for interfaces is modified to subtype
the first generated interface in the inheritance hierarchy.
- Dictionaries and typedef-like types are generated based on
whether they are used as they don't have compatibility data. In
order to determine this, whenever we generate a _RawType, we
mark it as used, and recursively generate the types needed.
- For each API within an interface, compat data in that interface
and its superinterfaces are used to determine if an API is
generated.
- In order to support the above changes, intermediate
representations for some members (attributes, fields, constants)
are added. There are other members that might be worth moving to
an IR, but that refactoring can be done in a future CL.

Closes a number of issues:

dart-lang#209
dart-lang#234
dart-lang#216
dart-lang#205
dart-lang#203
dart-lang#192

- Removes an unnecessary null-check
- Refactors getTypeForUnionCalculation to a shared
_getJSTypeEquivalent function
- Avoids shadowing an instance variable