dart2js: please validate positions #3675
Description
We should add an assertion that ensures that all tokens are found between the begin and end token of the current element.
The current API is quite subtle and requires compiler engineers to be very careful about calling Compiler.withCurrentElement when reporting an error on an element that is being inlined. So it is easy to forget that.
Also, as some tokens are synthesized by the parser, we should probably record the original token in synthesized tokens. This will make the proposed validation easier to implement.