dartdevc: eliminate the foreignJS extra magic

[sigmundch]

DDC has special logic to emit code differently when in the context of JS foreign functions. This has worked in the past because DDC is fairly local and all uses of JS foreign functions were within the SDK.

This special logic is hard to follow: reading the runtime libraries is ambiguous and confusing. Also the assumptions are changing:

• we are discussing the possiblity of implementing simple kernel inlining transformations, which will break the SDK-only assumption above
• In recent explorations with optimizing js_util methods, we have added lowering to helper methods and even to JS foreign calls. This lowering broke assumptions made by DDC and started triggering issues. For example see https://dart-review.googlesource.com/c/sdk/+/199540

To eliminate all this special magic we should instead:

• use helpers methods like typeRep<T> in the sdk libraries to remove the special magic.
• eventually reduce the use of large string-interpolated inputs to JS, instead adopt a more frugal use of JS like dart2js does.

[nshahan]

I recently refactored two of the methods that use large string-interpolated inputs to JS.
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