This commit allows compliance test-cases to be written that specify
source-map mappings between the source and generated code.
To check a mapping, add a `// SOURCE:` comment to the end of a line:
```
<generated code> // SOURCE: "<source-url>" <source code>
```
The generated code will still be checked, stripped of the `// SOURCE` comment,
as normal by the `expectEmit()` helper.
In addition, the source-map segments are checked to ensure that there is a
mapping from `<generated code>` to `<source code>` found in the file at
`<source-url>`.
Note:
* The source-url should be absolute, with the directory containing the
TEST_CASES.json file assumed to be `/`.
* Whitespace is important and will be included when comparing the segments.
* There is a single space character between each part of the line.
* Newlines within a mapping must be escaped since the mapping and comment
must all appear on a single line of this file.
PR Close#39939
9066ca9e92